diff --git a/mypy.ini b/mypy.ini
index 522d42e..8bd2791 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -1,5 +1,5 @@
 [mypy]
-python_version = 3.8
+python_version = 3.9
 warn_return_any = true
 warn_unused_configs = true
 warn_unused_ignores = true
diff --git a/pymars/_forward.py b/pymars/_forward.py
index 7ada1e0..74b3772 100644
--- a/pymars/_forward.py
+++ b/pymars/_forward.py
@@ -7,6 +7,7 @@
 """
 
 import logging
+from typing import Optional
 
 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, Optional[np.ndarray], 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[Optional[float], Optional[np.ndarray]]:
         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, Optional[BasisFunction]]]:
+        candidate_additions: list[tuple[BasisFunction, Optional[BasisFunction]]] = []
         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..d086c62 100644
--- a/pymars/_pruning.py
+++ b/pymars/_pruning.py
@@ -8,6 +8,7 @@
 """
 
 import logging
+from typing import Optional
 
 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, Optional[np.ndarray], 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[Optional[float], Optional[float], Optional[np.ndarray]]:
         """
         Computes GCV, RSS, and coefficients for a given subset of basis functions.
         Returns (gcv, rss, coeffs).
diff --git a/pyproject.toml b/pyproject.toml
index 07a6159..b96097b 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -102,6 +102,8 @@ exclude = [
 [tool.ruff.lint.per-file-ignores]
 # Allow print statements in demos
 "pymars/demos/*" = ["E501", "S101"]
+# Allow assert statements and long lines in tests (asserts are expected in tests)
+"tests/*" = ["S101", "E501"]
 
 [tool.ruff.lint.mccabe]
 # Unlike Flake8, default to a complexity of 10.
diff --git a/tox.ini b/tox.ini
index de1aa2c..0285f38 100644
--- a/tox.ini
+++ b/tox.ini
@@ -2,11 +2,20 @@
 envlist = py38, py39, py310, py311, py312
 isolated_build = true
 
+[gh-actions]
+python =
+    3.8: py38
+    3.9: py39
+    3.10: py310
+    3.11: py311
+    3.12: py312
+
 [testenv]
 deps = 
     pytest
     pytest-cov
     coverage[toml]
+    hypothesis
 commands = 
     pytest tests/ --cov=pymars --cov-report=term-missing --cov-report=html
     python -c "import coverage; cov = coverage.Coverage(); cov.load(); print(f'Coverage: {cov.report():.2f}%')"