[AIRFLOW-5251] add missing typing-extensions dep for py37

[houqp]

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Turns out Protocol is still experimental and not available in python 3.7's typing module. So typing-extensions is still required for py3.7.

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• Passes flake8

[ashb]

Oh yes, Protocol isn't in core until 3.8, and even then it's under typing, not typing_exetenions :(

Could you update the build matrix so that one of the jobs runs on Python 3.7 please?

And do you think it is worth keeping this to <3.8 and adding an import guard to the few places we use it:

try:
    from typing import Protocol
except ImportError:
    from typing_extensions import Protocol

possibly bundled up into a module somewhere in the airflow tree so that we can, for example, just do from airflow.typing import Protocol whereever we want to use it without worrying about the import guard everywhere.

[houqp]

Sure, I will make that change today :)

[houqp]

I have changed the mysql build to use python 3.7. @ashb @potiuk let me know if you prefer to create a new build matrix entry for 3.7 instead.

one surprise is some of the identity comparison behaviors have been changed in 3.7, so I had to put in python version check in unit test to change the assertion statement conditionally.

[ashb]

Thanks @houqp (for this and the related PR). Once this is green I'll merge.

[dstandish]

It seems this may be causing a problem with alembic. Since you run alembic from <root>/airflow directory, when it tries to import typing_extensions, it then tries to open typing, which resolves to this typing module and not the actual one.

That's probably confusing, but basically if I run alembic revision -m "something" I get import error from tying module.

If I run from repo root (specifying config file and changing script_location I don't get this error. (i get ERROR - Multiple heads are present;)