Fix macos homebrew python env

[saosebastiao]

This is a work in progress, intending to solve a problem with newer python requirements which don't allow user or system level dependencies.

I'm having multiple issues with nuttx that I'm not sure how to solve yet.

https://github.com/PX4/PX4-Autopilot/issues/23073

Solved Problem

This correctly implements a Python Virtual Environment which allows for local installation of all python dependencies. It updates CMakeLists.txt references to use the FindPython3 cmake module, which can correctly identify a virtual environment if it is sourced. In the root level CMakeLists.txt, it will mimic activation of the virtual environment via setting the $VIRTUAL_ENV environment variable.

Something in this merge request seems to be conflicting with the customized CMakeFiles.txt for our NuttX fork, and will result in build errors for any target built on nuttx. It seems as though certain libraries are not being built, and it's not clear to me what about the python virtual environment is causing this.

Additionally, the libuavcan submodule is using the old FindPythonInterp module which will not recognize virtual environments. When I looked at the git repo for libuavcan, it says that it is deprecated and should be replaced with https://github.com/dronecan/libcanard, so it probably means they won't be accepting any pull requests to fix that. I'm not sure what problems might arise from this, as I don't think I can even get to the point in the build process where that might be tested.

Some miscellaneous notes

I chose to use a hidden directory (.venv/) for the virtual environment, as this directory is often just extra noise in filesystem explorer apps. If necessary, we can use the more conventional and unhidden venv/.