Use Literal from typing_extensions (compatible with Python 3.7)

[MadLittleMods]

Use Literal from typing_extensions (compatible with Python 3.7)

Fix the following mypy errors when running mypy with Python 3.7:

synapse/config/_base.pyi:2: error: Module "typing" has no attribute "Literal"  [attr-defined]

Part of #15603

typing.Literal is only available in Python 3.8 or later, https://docs.python.org/3/library/typing.html#typing.Literal

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
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[MadLittleMods]

This PR is no longer necessary because we're now targeting Python 3.8 as the minimum version which is where typing.Literal was introduced.

But perhaps we want to do this solely to align our single typing.Literal usage to typing_extensions.Literal which we do everywhere else.

Or maybe we want to instead move everything to typing.Literal ⏩

Pokes in whatever direction welcome

[clokep]

I think move to typing.Literal ASAP when we drop 3.7 is the way to go