add Subquery class

[speezepearson]

Subquerys are new in 1.4 (relevant: #184). I used one for the first time today, and Mypy pointed out that it was missing from these type stubs.

I'm not an expert at SQL or sqlalchemy, so definitely open to feedback.

Thanks for your time!

Relevant links:

• SelectBase.subquery() docs
• AliasedReturnsRows impl
• Subquery impl

[CLAassistant]

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[speezepearson]

Closing because I'm not willing to grant @CLAassistant all the permissions it requires, and I bet the maintainers have better things to do than jump through whatever legal hoops are involved in finding an alternative solution.