Enable mypy strict mode

[rnestler]

Following up on #384 it would be good to track the progress towards mypy --strict mode. Strict mode is important to verify, that the type hints are correct, especially for a project which advertises itself as typed with the py.typed marker.

• 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 (#430)
• warn_redundant_casts = true
• warn_unused_ignores = true
• warn_return_any = true
• no_implicit_reexport = true
• strict_equality = true
• strict_concatenate = true