Revert "Remove unnecessary _PyCapsule class definition and import for clarity"

This reverts commit 0a10e48.

Author: kosiew

python/datafusion/user_defined.py

@@ -41,8 +41,13 @@
 from datafusion.expr import Expr
 
 if TYPE_CHECKING:
+    from _typeshed import CapsuleType as _PyCapsule
+
     _R = TypeVar("_R", bound=pa.DataType)
+else:
 
+    class _PyCapsule:
+        """Lightweight typing proxy for CPython ``PyCapsule`` objects."""
 
 
 class Volatility(Enum):