Support for Python packages, virtualenv

[martinhoyer]

The idea is that as a user, I want to be able specify list of python packages required for the test.
Prepare step would create a virtualenv in temporary dir with randomised name and install all the required packages.
Execute step would run tests within the created virtualenv.
Once finished, virtualenv can be destroyed.

Additional notes:

• Could also accept requirements.txt
• It would be great to be able to specify minimal python version.
  For example, a test could have python>=3.7 in metadata. tmt would check if valid interpreter exists on OS, if not try to install it with package manager and use it when creating virtualenv.
• Option to specify index with pypi.org as default?
• Option to update pip, wheel to latest (or >=?) before installing packages.