feat: allow pip to get default from toolchain config

[rickeylev]

This allows pip.parse to have python_version unspecified, getting the value from the
default configured by the python extension. This allows the pip integration code to
automatically match a dynamically computed default (i.e. one from python_version_env
or python_version_file). With the two in sync, this then allows pip to generate
appropriate code that relies on knowing the python version.

DO NOT MERGE, todo:

• update changelog
• misc cleanup

Work towards #3293