fix: Stop unconditionally importing contextvars (#6625)

Use the existing `_get_contextvars()` compatibility helper. The thread-local fallback is still needed for Python 3.6 because `contextvars` was added to the standard library in Python 3.7.

The `ImportError` is a regression introduced by 2604409. That commit ported code from a branch where the minimum supported Python version had already been raised to 3.7.

Author: alexander-alderman-webb

sentry_sdk/utils.py

@@ -1,5 +1,4 @@
 import base64
-import contextvars
 import copy
 import json
 import linecache
@@ -84,29 +83,6 @@
 
 _installed_modules = None
 
-_is_sentry_internal_task = contextvars.ContextVar(
-    "is_sentry_internal_task", default=False
-)
-
-# These exceptions won't set the span status to error if they occur. Use
-# register_control_flow_exception to add to this list
-_control_flow_exception_classes: "set[type]" = set()
-
-
-def is_internal_task() -> bool:
-    return _is_sentry_internal_task.get()
-
-
-@contextmanager
-def mark_sentry_task_internal() -> "Generator[None, None, None]":
-    """Context manager to mark a task as Sentry internal."""
-    token = _is_sentry_internal_task.set(True)
-    try:
-        yield
-    finally:
-        _is_sentry_internal_task.reset(token)
-
-
 BASE64_ALPHABET = re.compile(r"^[a-zA-Z0-9/+=]*$")
 
 FALSY_ENV_VALUES = frozenset(("false", "f", "n", "no", "off", "0"))
@@ -1468,6 +1444,26 @@ def _get_contextvars() -> "Tuple[bool, type]":
 Please refer to https://docs.sentry.io/platforms/python/contextvars/ for more information.
 """
 
+_is_sentry_internal_task = ContextVar("is_sentry_internal_task", default=False)
+
+# These exceptions won't set the span status to error if they occur. Use
+# register_control_flow_exception to add to this list
+_control_flow_exception_classes: "set[type]" = set()
+
+
+def is_internal_task() -> bool:
+    return _is_sentry_internal_task.get()
+
+
+@contextmanager
+def mark_sentry_task_internal() -> "Generator[None, None, None]":
+    """Context manager to mark a task as Sentry internal."""
+    token = _is_sentry_internal_task.set(True)
+    try:
+        yield
+    finally:
+        _is_sentry_internal_task.reset(token)
+
 
 def qualname_from_function(func: "Callable[..., Any]") -> "Optional[str]":
     """Return the qualified name of func. Works with regular function, lambda, partial and partialmethod."""