Add support for venvs.

[BeastLe9enD]

I have a python venv and in my opinion, there should be a function to load the venv into the pyo3 runtime. Something like

pyo3::set_venv("/path/to/your/venv");

would make things very easy. Or is there a simple way how this is possible that I'm missing?

[adamreichold]

Usually, activating the venv before building your project should be sufficient as we normally use the first python3 binary found on the path to determine the interpreter configuration.

[mobile-bungalow]

those of us on MacOs are still waiting on #1741. additionally, hot swapping venvs at runtime has more surface area than just updating the path as suggested in the linked issue. You also have to remove the sites-packages paths from preexisting venvs if they exist and account for platform differences in path name. This might actually be a small nice to have.