mypy: simplify configuration

mypy.ini

@@ -1,24 +1,10 @@
 [mypy]
+strict = True
 show_error_context = true
 show_column_numbers = true
 show_error_codes = true
 pretty = true
-
-# --strict options as of mypy 0.910
-warn_unused_configs = True
-disallow_any_generics = True
-disallow_subclassing_any = True
-disallow_untyped_calls = True
-disallow_untyped_defs = True
-disallow_incomplete_defs = True
-check_untyped_defs = True
-disallow_untyped_decorators = True
-no_implicit_optional = True
-warn_redundant_casts = True
-warn_unused_ignores = True
-warn_return_any = True
-implicit_reexport = False
-strict_equality = True
+warn_unreachable = True
 
 [mypy-ruamel.*]
 ignore_errors = True