Replace Python env managers with pet.el

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pyenv-mode, pyvenv, pipenv, uv-mode, conda, poetry... The :lang python module has a lot of virtualenv managers of varying quality that don't work very well together and have too many edge cases (see issues below). They will be replaced with [pet](https://github.com/wyuenho/emacs-pet) soon.

This issue combines all of the following:

- https://github.com/doomemacs/core/pull/8692
- https://github.com/doomemacs/core/issues/8110
- https://github.com/doomemacs/core/issues/7520
- https://github.com/doomemacs/core/issues/5826
- https://github.com/doomemacs/core/issues/5715
- https://github.com/doomemacs/core/issues/1666
- https://github.com/doomemacs/core/issues/736