Replace flit by flit_core in build-system

[tjni]

flit_core is the lighter weight PEP 517 build backend. This lets us build the package using a different front-end, such as build, without pulling in all of flit.

[codecov]

Codecov Report

Merging #205 (35048fa) into main (735154e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #205   +/-   ##
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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           

[Mariatta]

Thank you. Should we pin the minimum version of flit that supports this?

[tjni]

Great question. I locally was able to build the package using flit_core 2.0, which is the earliest released PyPI version, so I don't think it's technically necessary, but I don't mind increasing it either.

[brettcannon]

Thanks, @tjni ! Since you verified a lower-bound isn't necessary I didn't worry about setting one.