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/pytester.py

@@ -1169,8 +1169,10 @@ class Testdir:
 
         popen = subprocess.Popen(cmdargs, stdout=stdout, stderr=stderr, **kw)
         if stdin is Testdir.CLOSE_STDIN:
+            assert popen.stdin is not None
             popen.stdin.close()
         elif isinstance(stdin, bytes):
+            assert popen.stdin is not None
             popen.stdin.write(stdin)
 
         return popen