ENH: Consider delegating to nox

[HaoZeke]

On the Scientific Python Discord it was noted that perhaps existing tools like tox could be used for managing the environments asv needs.

Currently this is bundled into asv itself, and works alright. However, a possible cleanup would be delegating to nox (which also supports conda and virtualenv) and might make it easier to add more flexibility into the environment management, while also being easier to maintain.

[mattip]

Before work on this, we should have a downstream user clearly state why virtualenv does not suit what they are trying to do with ASV.

[trexfeathers]

A team I work in adopted Nox for several projects, but we have since decided to switch to Tox. It is easier to write things in Nox, but we felt that the 10x popularity of Tox promised a healthier future and easier collaboration with other developers.

I don't have a strong opinion myself; just thought it was worth sharing.