Consistency: apply codestyle formatting to docstring code snippets

Consistency: apply codestyle formatting to docstring code snippets

### Description
This is a pedantic/consistency follow-up from #1219: that change applied some `black` formatting to two code snippets, and this change applies that formatting to the remaining snippets in the codebase.

For each snippet, I extracted the code and applied formatting using `black` v23.1.0, then placed the results back into the source.

In one case there was an associated 'output' block, and in that case I re-ran the snippet code to update the results of that output too (this included a [change-in-output](https://github.com/sqlalchemy/alembic/compare/main...openculinary:alembic:docstrings/snippet-format-consistency?expand=1#diff-bf4756660cdb31ee8566a2cff72526671356c38a725195723ce0e65e6c11e6cfR124) thanks to a [bugfix since the snippet was written](https://github.com/sqlalchemy/alembic/commit/bc6971aa4abdafb7e1a1123c26a373cc25a34ca9), by the looks of it).

### Checklist

This pull request is:

- [x] A documentation / typographical error fix

Closes: #1220
Pull-request: https://github.com/sqlalchemy/alembic/pull/1220
Pull-request-sha: cd65a453192546a660d6f5ebad5af08bb1c72af4

Change-Id: I6758445633c364c8fb2f4d8376d83607430a36d6