fix mypy type error in meta.py

[cosmicexplorer]

Problem

pants.util.meta.frozen_after_init has a couple mypy type checking errors, specifically at:

pants/src/python/pants/util/meta.py

Lines 140 to 144 in 00c4f8c

	setattr(cls, frozen_after_init.sentinel_attr, True)
	return cls
	frozen_after_init.sentinel_attr = '_frozen_after_init'

This caused a failure on master, see https://travis-ci.org/pantsbuild/pants/jobs/600466692.

Solution

• Add # type: ignore to the offending lines until improve ergonomics of checking for @union types with new @decorated_type_checkable decorator #8496 is merged.

Result

CI is unbroken!

[Eric-Arellano]

Hm any ideas how this slipped past CI? Did we merge on red?

Thank you for fixing this!

[cosmicexplorer]

Yes, I merged on red for #8181, and made a change I didn't realize would break the type checking (the precise change that caused this error, see 00c4f8c#diff-ec31f60c86dc5b36ed1ab04df574c245).