Skip the default heroku-postgresql addon if a Postgres DB driver isn't installed

[edmorley]

Currently the Python buildpack's bin/release step sets a default heroku-postgresql addon if the app has a manage.py script.

However for use-cases where either no database is being used, or a non-Postgres DB is used, this default addon at best is unused and at worst causes confusion.

To resolve this the buildpack could check that the installed Python packages list includes psycopg2/...

This would be similar to the approach taken by the Ruby buildpack.

For more background see the discussion in: https://github.com/heroku/heroku-buildpack-python/pull/1056

[git2gus[bot]]

This issue has been linked to a new work item: W-8098780

[git2gus[bot]]

This issue has been linked to a new work item: W-8098791