Fix 'TODO: make sure all non-optional tagged arguments were seen' issue in sifter\grammar\rule.py

[CodeFactor](https://www.codefactor.io/repository/github/python-sifter/sifter3) found an issue: TODO: make sure all non-optional tagged arguments were seen

It's currently on:
[sifter\grammar\rule.py:96
](https://www.codefactor.io/repository/github/python-sifter/sifter3/source/master/sifter/grammar/rule.py#L96)Commit 6e6601ee7d45643e613c73a1db22a6d81d4b5942