support paths in file_template

[zzzeek]

Discussed in #1772

if I make a file_template like this:

file_template = %%(year)d/%%(month).2d/%%(day).2d_%%(hour).2d%%(minute).2d%%(second).2d_%%(rev)s_%%(slug)s

it should Just Work

so this means:

1. in script/base.py in generate_revision() there should be a self._ensure_directory() for the final calculated path of the output file
2. the alembic.ini.mako / pyproject.toml.mako templates should include an additional example with a line like the above, as well as in tutorial.rst editing-the-ini-file
3. the bulletpoint in editing-the-ini-file for file_template also needs a new paragraph /example for this with a .. versionadded:: directive afterwards.
4. all of 2,3 above also need to mention that recursive_version_locations=true must be set
5. tests in tests/test_script_produciton.py. generation of new file with directory scheme, use a mocked datetime, check for directory path exists, then test creating a ScriptDirectory against it and the new file is loctable

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the main branch:

support paths in file_template https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/6536