Add some DB API 2.0 protocols and types - #6643

The DB API 2.0 spec contains more mandatory items as well as a bunch of optional or "extension" items. These can be added as needed. The definitions above are useful for the SQLAlchemy stubs.

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

How do you actually import these in a project?

@clbarnes You need to use:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from _typeshed.dbapi import ...

Then you can use them either by quoting or when using from __future__ import annotations.