Bump mypy version to 0.981

[tetsuok]

This PR bumps mypy version to 0.981. This is to remove "ignore" comments to silence mypy.

• The version includes a fix for abstract dataclass inheritance gives Only concrete class can be given where "Type[Abstract]" is expected python/mypy#5374. This PR removes the comments to silence mypy due to the issue.
• The version has a fix for Support recursive types python/mypy#731. The fix is enabled when running mypy with --enable-recursive-aliases (link to blog post), but this PR doesn't enable the fix, deferring to a separate PR. This is because using the option causes different 91 errors. It seems better to tackle enabling the option in a separate PR.
• The version doesn't have a fix for Use case for typing.Type with abstract types python/mypy#4717, unfortunately. mypy still complains when the comments were removed.

[odashi]

@tetsuok Maybe this update resolves some issues tagged by 4717 as well as 5374.

[odashi]

dummy

[tetsuok]

@odashi Issues on python/mypy#4717 still persist with this update.