python module: stop using distutils "link to libpython" probe on recent python

On python >=3.8, this information is expected to be encoded in the
sysconfig vars.

In distutils, it is always necessary to link to libpython on Windows;
for posix platforms, it depends on the value of LIBPYTHON (which is the
library to link to, possibly the empty string) as generated by
configure.ac and embedded into python.pc and python-config.sh, and then
coded a second time in the distutils python sources.

There are a couple of caveats which have ramifications for Cygwin and
Android:

- python.pc and python-config.sh disagree with distutils when python is
  not built shared. In that case, the former act the same as a shared
  build, while the latter *never* links to libpython

- python.pc disagrees with python-config.sh and distutils when python is
  built shared. The former never links to libpython, while the latter do

The disagreement is resolved in favor of distutils' behavior in all
cases, and python.pc is correct for our purposes on python 3.12; see:
https://github.com/python/cpython/pull/100356
https://github.com/python/cpython/pull/100967

Although it was not backported to older releases, Cygwin at least has
always patched in a fix for python.pc, which behavior is now declared
canonical. We can reliably assume it is always correct.

This is the other half of the fix for #7702

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