Add a --strict option to the Mypy checks

It must close pypa#10076, to include all the strict flags from Mypy to be used on pypa/pip.

.pre-commit-config.yaml

@@ -60,7 +60,7 @@ repos:
   hooks:
   - id: mypy
     exclude: tests
-    args: ["--pretty"]
+    args: ["--pretty", "--strict"]
     additional_dependencies: ['nox==2020.12.31']
 
 - repo: https://github.com/pre-commit/pygrep-hooks