Fix repr and add tests

[pamelafox]

This PR adds test for SQLAlchemy's repr and fixes its handling of the no default engine scenario.

This fix assumes we want engine() to continue to throw a KeyError in that scenario (i.e. all handling is in repr itself).

• fixes SQLAlchemy.__repr__ raises KeyError when the default engine is not set. #1295

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.