Require typing-extensions>=4.9.0 for python older than 3.11

[francesco-ballarin]

Fixes #177.

I've got a CI that runs on a docker image with typing_extensions 4.4.0 already installed, and received failures with

/usr/local/lib/python3.10/dist-packages/plum/util.py:5: in <module>
    from typing_extensions import deprecated
E   ImportError: cannot import name 'deprecated' from 'typing_extensions' (/usr/local/lib/python3.10/dist-packages/typing_extensions.py)

Seems like deprecated was introduced in 4.5.0, and improved in 4.9.0, see
https://typing-extensions.readthedocs.io/en/latest/#deprecated
so I chose the latter as lower bound.

[PhilipVinc]

Great catch, thanks!

[coveralls]

Pull Request Test Coverage Report for Build 9827132270

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.763%

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Totals
Change from base Build 9819027928:	0.0%
Covered Lines:	1265
Relevant Lines:	1268

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💛 - Coveralls