Set a maximum version of flit_core for build requirements

[takluyver]

The plan for Flit 4 is to drop support for the [tool.flit.metadata] project description in favour of the [project] table, which is now standardised across different tools.

More generally, I advise packages to always restrict the dependency to exclude the next major version of flit_core, so that I can make breaking changes. You can see the version ranges recommendations here: https://flit.pypa.io/en/stable/pyproject_toml.html#build-system-section

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (7ec51fc) to head (5d06df7). Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #211   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines          471       471           
  Branches        87        87           
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  Hits           471       471           

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