Loosen restrictions on mypy (jaraco/skeleton#136)

Based on changes downstream in setuptools.

mypy.ini

@@ -1,5 +1,14 @@
 [mypy]
-ignore_missing_imports = True
-# required to support namespace packages
-# https://github.com/python/mypy/issues/14057
+# Is the project well-typed?
+strict = False
+
+# Early opt-in even when strict = False
+warn_unused_ignores = True
+warn_redundant_casts = True
+enable_error_code = ignore-without-code
+
+# Support namespace packages per https://github.com/python/mypy/issues/14057
 explicit_package_bases = True
+
+# Disable overload-overlap due to many false-positives
+disable_error_code = overload-overlap