Evaluate if we still need to pin virtualenv<20

[ianpittwood]

I noticed that we use virtualenv<20 in our base image without further context. I suspect this was put in due to an older issue with Debian-based OSes like this: python/mypy#10407

We should evaluate whether we can remove this line and move forward with using modern versions of virtualenv in our base image.

[aronatkins]

The virtualenv<20 constraint was introduced because early versions had some problems. Internally, we removed some of those constraints around the time of 20.0.20. CC @mmarchetti for more context.

[mmarchetti]

Virtualenv 20 was a rewrite, which naturally introduced many breaking changes and bugs.

The original issue was https://github.com/rstudio/connect/issues/16474. After working around that on the Connect side, we encountered an issue with how environments are built: https://github.com/rstudio/connect/issues/16569. We documented a requirement for virtualenv<20 and configured all of our docker images and test environments that way. A few months later, once virtualenv had stabilized, we removed the constraint.