Fix 'TODO: make sure the specified pattern is allowed by the standard' issue in sifter\grammar\comparator.py

[CodeFactor](https://www.codefactor.io/repository/github/python-sifter/sifter3) found an issue: TODO: make sure the specified pattern is allowed by the standard

It's currently on:
[sifter\grammar\comparator.py:32
](https://www.codefactor.io/repository/github/python-sifter/sifter3/source/master/sifter/grammar/comparator.py#L32)Commit 6e6601ee7d45643e613c73a1db22a6d81d4b5942