Allow upgrading to latest

[jayvdb]

Now that this plugin supports tox-venv, it could be used to upgrade the pip inside the venv. See tox-dev/tox-venv#25 (comment) That isnt usually required, as tox without tox-venv will use virtualenv which does this upgrade automatically.

This might also be implemented as part of 'ranges' #12 , if it uses normal specifiers which can include >19, which will typically cause pip to install the latest.

[pglass]

This should be addressed in #16. We'll be able to use pip_version = pip to install latest (equivalent to a pip install -U pip) or use pip_version = pip>=19.1 or similar.

[jayvdb]

I think this (upgrade to latest) needs to be possible with any extra dependencies inside the venv , such as the dependency on packaging in the venv.

[pglass]

Yeah, please look closer at the PR. packaging is a dev/test dependency only.