fix: don't need a python interpreter when not having pypi dependencies.

[ruben-arts]

Fixes #1232

Now we do two things if there is no pypi-dependencies:

• Check if we need to remove some site-packages.
• Skip everything that requires to call the python interpreter.

To not make the test abonormally slow we "abuse" one of the tests to try to install python 2.7 which will not work if we by accident try to run any uv related work.

[wolfv]

Looks good!