Static dispatch of alembic comparators

[DanCardin]

Fixes https://github.com/olirice/alembic_utils/issues/80

So there are a few things i'll point out:

• old versions of black are broken if you can't pin click (https://github.com/psf/black/issues/2964). I'm not familiar with pre-commit, so i just opted to upgrade black. That caused some docstring normalization which I tried to include as a separate first commit. But otherwise I believe it's an option you could theoretically turn off. Or alternatively I could figure out how to avoid the upgrade.
• I got a mypy error on your env.py. Dunno if it's because i ended up with a different version of mypy than you since it's not pinned in the dev deps. I fixed that and that's commit number 2.
• Finally the actual body of work is commit number 3. The only meaningful test fallout was that duplicate entity registration just doesn't happen now (since you asked for me to deduplicate), so i opted to remove that exception. I could instead raise that exception inside the body of the register function, if you preferred.

[DanCardin]

the net -1 line of code cov looks like is because there's no existing coverage of exclude_schemas which your comments imply is deprecated anyway. I can perhaps add a test for it, if you think that's important

[olirice]

the net -1 line of code cov looks like is because there's no existing coverage of exclude_schemas which your comments imply is deprecated anyway. I can perhaps add a test for it, if you think that's important

Yeah I've been meaning to remove it. No worries about the coverage line