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 https://github.com/bazel-contrib/rules_python/issues/3293