bpo-45873: Get rid of bootstrap_python (#29717)

Instead we use $(PYTHON_FOR_REGEN) .../deepfreeze.py with the
frozen .h file as input, as we did for Windows in bpo-45850.

We also get rid of the code that generates the .h files
when make regen-frozen is run (i.e., .../make_frozen.py),
and the MANIFEST file.

Restore Python 3.8 and 3.9 as Windows host Python again

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>