bpo-22490: Remove __PYVENV_LAUNCHER__ from environment during launch (GH-9516)

* bpo-22490: Remove "__PYVENV_LAUNCHER__" from the shell environment on macOS

This changeset removes the environment varialbe "__PYVENV_LAUNCHER__"
during interpreter launch as it is only needed to communicate between
the stub executable in framework installs and the actual interpreter.

Leaving the environment variable present may lead to misbehaviour when
launching other scripts.

* Actually commit the changes for issue 22490...

* Correct typo

Co-Authored-By: Nicola Soranzo <nicola.soranzo@gmail.com>

* Run make patchcheck

Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>
Co-authored-by: Nicola Soranzo <nicola.soranzo@gmail.com>