mypy: enable warnings for unnecessary cast/ignore

This doesn't have any effect currently, but it's good to know if
anything ever becomes unnecessary.

tildes/mypy.ini

@@ -3,6 +3,8 @@ mypy_path = /opt/tildes/stubs/
 disallow_untyped_defs = true
 ignore_missing_imports = true
 no_implicit_optional = true
+warn_redundant_casts = true
+warn_unused_ignores = true
 
 [mypy-tests.*]
 disallow_untyped_defs = false