PR summary

As a followup to#28856, we can now do some final cleanup:

• drop all the old Python → C converters, and other helpers
• also movepy_converters_11.{cpp,h} intopy_converters.{cpp,h}
• remove NumPy as a build dependency

PR checklist

• "closes #0000" is in the body of the PR description tolink the related issue
• new and changed code istested
• [n/a]Plotting related features are demonstrated in anexample
• New Features andAPI Changes are noted with adirective and release note
• Documentation complies withgeneral anddocstring guidelines