Update mypy and remove mypy_hack

[akawasaki]

Description

Updated mypy to latest version to revert some hacks done for #120

• Updated mypy to 0.971
• revert mypy hacks done for #120
• Resolve mypy error Function does not return a value
• Some tests improvements

[akawasaki]

@ojii please take a look at this PR 🙏

[dimaqq]

Resolve mypy error Function does not return a value

Apparently there's a flag for that, see https://github.com/python/mypy/pull/13219