diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 00f97d3..5dae1c3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -11,17 +11,21 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.8, 3.9, '3.10', '3.11', '3.12']
+        python-version: [3.8, 3.9, '3.10']
 
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v4
     - name: Set up Python ${{ matrix.python-version }}
-      uses: actions/setup-python@v6
+      uses: actions/setup-python@v4
       with:
         python-version: ${{ matrix.python-version }}
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
+        # Install importlib-metadata backport for Python < 3.10
+        if [[ "${{ matrix.python-version }}" == "3.8" ]] || [[ "${{ matrix.python-version }}" == "3.9" ]]; then
+          pip install importlib-metadata
+        fi
         pip install tox tox-gh-actions
     - name: Test with tox
       run: tox -e py
@@ -29,9 +33,9 @@ jobs:
   lint:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v4
     - name: Set up Python
-      uses: actions/setup-python@v6
+      uses: actions/setup-python@v4
       with:
         python-version: '3.11'
     - name: Install dependencies
@@ -44,9 +48,9 @@ jobs:
   type-check:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v4
     - name: Set up Python
-      uses: actions/setup-python@v6
+      uses: actions/setup-python@v4
       with:
         python-version: '3.11'
     - name: Install dependencies
@@ -59,9 +63,9 @@ jobs:
   coverage:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v4
     - name: Set up Python
-      uses: actions/setup-python@v6
+      uses: actions/setup-python@v4
       with:
         python-version: '3.11'
     - name: Install dependencies
@@ -80,9 +84,9 @@ jobs:
   format:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v4
     - name: Set up Python
-      uses: actions/setup-python@v6
+      uses: actions/setup-python@v4
       with:
         python-version: '3.11'
     - name: Install dependencies
diff --git a/pymars/_compat.py b/pymars/_compat.py
new file mode 100644
index 0000000..bf7d3ec
--- /dev/null
+++ b/pymars/_compat.py
@@ -0,0 +1,6 @@
+try:
+    import importlib.metadata as importlib_metadata  # type: ignore
+except Exception:
+    import importlib_metadata  # type: ignore
+
+__all__ = ("importlib_metadata",)
diff --git a/pymars/_forward.py b/pymars/_forward.py
index 7ada1e0..763efc7 100644
--- a/pymars/_forward.py
+++ b/pymars/_forward.py
@@ -7,6 +7,7 @@
 """
 
 import logging
+from typing import Union
 
 import numpy as np
 
@@ -53,7 +54,7 @@ def __init__(self, earth_model: Earth):
 
     def _calculate_rss_and_coeffs(
         self, B_matrix: np.ndarray, y: np.ndarray, *, drop_nan_rows: bool = True
-    ) -> tuple[float, np.ndarray | None, int]:
+    ) -> tuple[float, Union[np.ndarray, None], int]:
         if B_matrix is None or B_matrix.shape[1] == 0:
             mean_y = np.mean(y)
             rss = np.sum((y - mean_y)**2)
@@ -225,7 +226,7 @@ def run(self, X_fit_processed: np.ndarray, y_fit: np.ndarray,
 
         return self.current_basis_functions, self.current_coefficients
 
-    def _calculate_gcv_for_basis_set(self, basis_functions: list[BasisFunction]) -> tuple[float | None, np.ndarray | None]:
+    def _calculate_gcv_for_basis_set(self, basis_functions: list[BasisFunction]) -> tuple[Union[float, None], Union[np.ndarray, None]]:
         if not basis_functions:
             # This implies an intercept-only model for GCV calculation purposes
             rss_intercept_only = np.sum((self.y_train - np.mean(self.y_train))**2)
@@ -348,8 +349,8 @@ def _get_allowable_knot_values(self, X_col_original_for_var: np.ndarray, parent_
             minspan_countdown = max(0, minspan_abs - 1)
         return np.array(final_allowable_knots)
 
-    def _generate_candidates(self) -> list[tuple[BasisFunction, BasisFunction | None]]:
-        candidate_additions: list[tuple[BasisFunction, BasisFunction | None]] = []
+    def _generate_candidates(self) -> list[tuple[BasisFunction, Union[BasisFunction, None]]]:
+        candidate_additions: list[tuple[BasisFunction, Union[BasisFunction, None]]] = []
         for parent_bf in self.current_basis_functions:
             if parent_bf.degree() + 1 > self.model.max_degree: continue
             parent_involved_vars = parent_bf.get_involved_variables()
diff --git a/pymars/_pruning.py b/pymars/_pruning.py
index 07bb477..a54dfd3 100644
--- a/pymars/_pruning.py
+++ b/pymars/_pruning.py
@@ -8,6 +8,7 @@
 """
 
 import logging
+from typing import Union
 
 import numpy as np
 
@@ -38,7 +39,7 @@ def __init__(self, earth_model: Earth):
         self.best_basis_functions_so_far: list[BasisFunction] = []
         self.best_coeffs_so_far: np.ndarray = None
 
-    def _calculate_rss_and_coeffs(self, B_matrix: np.ndarray, y_data: np.ndarray) -> tuple[float, np.ndarray | None, int]:
+    def _calculate_rss_and_coeffs(self, B_matrix: np.ndarray, y_data: np.ndarray) -> tuple[float, Union[np.ndarray, None], int]:
         """
         Calculates RSS, coefficients, and num_valid_rows, considering NaNs in B_matrix.
         y_data is assumed finite.
@@ -97,7 +98,7 @@ def _build_basis_matrix(self, X_data: np.ndarray, basis_functions: list[BasisFun
 
     def _compute_gcv_for_subset(self, X_fit_processed: np.ndarray, y_fit: np.ndarray,
                                 missing_mask: np.ndarray, X_fit_original: np.ndarray,
-                                basis_subset: list[BasisFunction]) -> tuple[float | None, float | None, np.ndarray | None]:
+                                basis_subset: list[BasisFunction]) -> tuple[Union[float, None], Union[float, None], Union[np.ndarray, None]]:
         """
         Computes GCV, RSS, and coefficients for a given subset of basis functions.
         Returns (gcv, rss, coeffs).