🆙 Set minimum Python version for plum to Python 3.8

[gabrieldemarmiesse]

Python 3.7 end of support for security bugs was the 5th of june 2023.

Some notes here:

• This allows the python version in pre-commit to be the same as the one installed for testing of plum (3.8) because before, the pre-commit python version was 3.8 and the minimum python version for plum was 3.7.
• Since typing.Literal is available in python 3.8, I think plum.Val can be now deprecated. I made the related changes.
• get_args and get_origin can now be imported normally.

[coveralls]

Pull Request Test Coverage Report for Build 5878268210

• 4 of 4 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 5864280507:	0.0%
Covered Lines:	922
Relevant Lines:	922

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💛 - Coveralls

[wesselb]

This is great! Removes a whole bunch of annoying boilerplate. Thanks!! :)