Fix alembic ini finder

[SpacemanPaul]

Reason for this pull request

The alembic.ini file was previously kept in the root directory of the repo, which is fine if installed locally with pip install -e . but doesn't work when installing from PyPI.

Proposed changes

• Move alembic.ini to the drivers/postgis directory.

  • [x] Tests added / passed
  • [x] Fully documented, including docs/about/whats_new.rst for all changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.43%. Comparing base (dbed0ac) to head (f8107ee).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1597 +/- ## ============================================ Coverage 85.43% 85.43% ============================================ Files 140 140 Lines 15496 15496 ============================================ Hits 13239 13239 Misses 2257 2257 ```

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