A few linting fixes

Add some Python 3.8 type: ignores; all are already fixed in the next
mypy release, so can be removed once we upgrade.

Also move some flake8 ignores which seem to have changed places.

src/_pytest/python_api.py

@@ -552,7 +552,7 @@ def raises(
 
 
 @overload  # noqa: F811
-def raises(
+def raises(  # noqa: F811
     expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
     func: Callable,
     *args: Any,