Introduce stricter mypy options

This is a first step according to the documentation https://mypy.readthedocs.io/en/stable/existing_code.html#getting-to-strict

Author: kbaikov

mypy.ini

@@ -1,5 +1,9 @@
 [mypy]
 
+warn_unused_configs = True
+warn_redundant_casts = True
+warn_unused_ignores = True
+
 [mypy-test.*]
 disallow_untyped_defs = True
 disallow_untyped_calls = True