Update `os.environ['PYTHONPATH']` with the contents of `pythonpath`

[xmo-odoo]

What's the problem this feature will solve?

Basically what #9134 solved, with the addition of subprocesses being spawned by the SUT or being the SUT itself (I regularly use pytest for non-python utilities and the likes)

Describe the solution you'd like

That at the same time the content of pythonpath is prepended to sys.path, it's also prepended to os.environ['PYTHONPATH']

Alternative Solutions

python-dotenv has understandably rejected special support for PYTHONPATH (theskumar/python-dotenv#36). Though I guess pytest-dotenv might be open to that? I haven't found any mention of that on their bug tracker.

[RonnyPfannschmidt]

has to be rejected, i can think of at least 10 integration testsuites this one would break right off the bat

[nicoddemus]

To complement: the pythonpath config variable is just meant to help pytest import modules, so users not using the standard development workflow can easily help pytest import their test files. It is not meant as an actual config variable to update also environment variables. Doing so would definitely have consequences in existing test suites, either breaking immediately, or producing confusing errors hard to debug.

You can accomplish the same with an auto-use fixture in your conftest.py file.