Docs: Remove recommendation to install virtualenv separately
When this library drops Python 2 support, recommend that users use python -m venv rather than installing virtualenv separately.
Hmmm, this is a little weird, as the virtualenv library really isn't needed for supported Python 3.x versions: python -m venv works fine for them. It is a "works everywhere" recommendation as long as we are supporting Python 2.7, though.