Enable check_untyped_defs mypy option for src/

This option checks even functions which are not annotated. It's a good
step to ensure that existing type annotation are correct.

In a Pareto fashion, the last few holdouts are always the ugliest,
beware.

src/_pytest/python_api.py

@@ -508,7 +508,7 @@ def approx(expected, rel=None, abs=None, nan_ok=False):
     __tracebackhide__ = True
 
     if isinstance(expected, Decimal):
-        cls = ApproxDecimal
+        cls = ApproxDecimal  # type: Type[ApproxBase]
     elif isinstance(expected, Number):
         cls = ApproxScalar
     elif isinstance(expected, Mapping):
@@ -534,7 +534,7 @@ def _is_numpy_array(obj):
     """
     import sys
 
-    np = sys.modules.get("numpy")
+    np = sys.modules.get("numpy")  # type: Any
     if np is not None:
         return isinstance(obj, np.ndarray)
     return False