Provide a way to detect if Python is running from a pup installation

[carlosperate]

I'm looking into a simple way to fix this issue:

• Issue launching virtualenv module isolated from user's environment mu#1954

Basically people "pip install mu-editor" in their system Python, and in many Linux distros like Debian and derivatives, that will install any dependencies not already met in the "user site-packages".

Then Mu tries to launch Python with the -I (isolated) flag, for things like creating a venv and install pacakges, and it's not able to see packages in the "user site-packages", only the system ones.

Basically, I think we should only use the -I flag within Mu if it can detect it's running from a pup installer. So that if a user has installed it via pip or any other package manager, then it run with the same settings that are launching python -m mu.

@tmontes would there be a simple way pup can provide a quick check for this? Maybe an environmental variable that is set by pup?