Use plain python venv instead of virtualenv

[igordertigor]

The package was using virtualenv as an unlisted dependency. I'm switching here to using python -m venv instead, which appears to be part of the standard library. Without this, try is broken for me.

[timofurrer]

Let me give this a shot later today before I merge it :) But looks great!

[Bobronium]

@timofurrer, any updates on this?

Even if virtualenv is present, pip it for some reason creates venv with outdated pip which produces warnings.

Workaround:

1. install pipx via pipx-in-pipx

   pip install pipx-in-pipx

2. Install trypackage and virtualenv in isolated venvs:

   pipx install trypackage
   pipx install virtualenv

3. Bonus: upgrade bundled libs for virtualenv (gets rid of old pip warnings)

   virtualenv  --upgrade-embed-wheels