Document metamist's required Python version in setup.py

[jmarshall]

I've long run metamist (and the rest of my CPG work) on Python 3.10, but at least since PR #861 this no longer works — as StrEnum was introduced in Python 3.11:

ImportError: cannot import name 'StrEnum' from 'enum' (/opt/homebrew/Cellar/python@3.10/3.10.14_1/Frameworks/Python.framework/Versions/3.10/lib/python3.10/enum.py)

This is mentioned in docs/installation.md, in which PR #696 bumped the documented requirement from 3.10 to 3.11.

This adds a mention in the canonical build-system place. (pyproject.toml would be even more canonical, but we don't use a [project] section there, so setup.py is the canonical place for this project.)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.54%. Comparing base (216bb11) to head (5f8b938).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #881   +/-   ##
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  Coverage   80.54%   80.54%           
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  Files         172      172           
  Lines       14641    14641           
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  Hits        11793    11793           
  Misses       2848     2848           

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