Fix alembic incompatibility with sqlalchemy < 1.3.11

[elainearbaugh]

Also adds a unit test specifically targeting these sqlalchemy versions (this test fails without the fix). Not sure if this is the best way to check for this attribute so lmk if there's a more elegant way to do it

Fixes#269

• Tests pass
• [n/a] Appropriate changes to README are included in PR

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[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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[IlyaFaer]

@elainearbaugh, the PR looks okay to me. They only thing needed is for you to sign the CLA agreement:
https://github.com/googleapis/python-spanner-sqlalchemy/blob/main/CONTRIBUTING.md#contributor-license-agreement
Then we can merge it.

[elainearbaugh]

@IlyaFaer thanks! I just got the CLA signed this week, working through the other issues now

[elainearbaugh]

@IlyaFaer resolved the commit issues as well so if you could allow running workflows and re-review that'd be awesome! Sorry for being slow, it took a while for my org to get the CLA through

[elainearbaugh]

I'm not sure what's going on with the Kokoro failure--seems like it hit a "ERROR: Aborting VM command due to timeout of 10800 seconds" (3 hours)? I added some additional testing but it definitely doesn't take 3 hours running locally...I don't think I have the ability to rerun the tests if it's a flake so not sure what my next steps should be here

[olavloite]

Closing, as this seems to have been implemented through another PR (and this PR is also very old)