Pyre Failing on master branch

[lisroach]

Pyre failed due to a type error was found in fixit/rules/cls_in_classmethod.py:286:26 and that's related to the new LibCST release.
https://github.com/Instagram/Fixit/runs/2271833508
The same issue happens on master branch and it's better to fix it before merging new PRs.
FlattenSentinel is added in LibCST 0.3.18 and we need to update the type annotation of report().

Fixit/fixit/common/base.py

Line 120 in 1d1cfe9

replacement: Optional[Union[cst.CSTNode, cst.RemovalSentinel]] = None,

Originally posted by @jimmylai in #182 (comment)

[lisroach]

Fixed with #187