[pip] work inside a virual environment

[lypwig]

Some issues may appear due to the user's pip environment.

For instance some installed dependencies may cause troubles (see #122). flatpak-pip-generator.py will even not work at all if PIP_REQUIRE_VIRTUALENV is set to true and the script runs outside a venv.

This could be fixed if flatpak-pip-generator.py creates a temporary virtual environment to work on, ensuring that it always run in the same, clean environment.

[gasinvein]

Since the pip helper uses pip itself to resolve dependencies, it's a good idea to run the helper in flatpak app/SDK environment, so packages installed on host won't "leak" into generated dependency tree.

[muelli]

It's now possible to run in a flatpak container, e.g. --runtime org.gnome.Sdk//3.38. That addresses this issue, no?