Don't fail on mypy-style '# type: ignore[code, ...]' comments

[udim]

Currently a comment like # type: ignore[assignment] generates the error:

Invalid type comment: ignore[assignment] [invalid-type-comment]
  Name 'ignore' is not defined
  Name 'assignment' is not defined

I would expect pytype to consider such a comment as a plain # type: ignore.

ref: https://mypy.readthedocs.io/en/latest/error_codes.html#silencing-errors-based-on-error-codes

[rchen152]

Hmm, I'm not sure if we'll support this mypy-specific feature, since pytype has its own way of silencing errors (https://google.github.io/pytype/user_guide.html#silencing-errors), but at a minimum we can change the # type: ignore regex to accept an error code after the ignore.

[dumblob]

I'd advocate at least for not reporting it as an error, but a warning - i.e. not hindering execution (i.e. not requiring any code changes).