make `PythonParser` generic around cache mode, not its functions

[samuelcolvin]

Should make it easier to make PythonParser generic around more things.

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #79 will not alter performance

_{Comparing PythonParser-generic (875c58f) with main (efad44e)}

Summary

✅ 59 untouched benchmarks

[codecov[bot]]

Codecov Report

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

:loudspeaker: Thoughts on this report? Let us know!