requirement: add pour_bottle? method.

Allows disabling bottles from requirements.
formula_installer: check pour from requirements.
python_dependency: only pour with system python.

This is as Python bindings are not build in a way that can have
system or Homebrew Python used interchangeably.
python: support bottling.

Move a bunch of site_packages stuff to post_install.
python3: support bottling.

Move a bunch of site_packages stuff to post_install.

Closes #27968.