Riot

Fixes an issue in which riot generated a requirements lockfile for a Venv instance other than the one it was running tests for. This behavior was happening due to logic that skipped Venv instances with pkgs == None while preparing the environment, but not while running tests. The fix is to stop riot from ignoring pkgs-less Venvs in all cases.

Bug Fixes

- Avoid using latests versions of `virtualenv` since it has dropped support for Python 2.7.

New Features

- Adds the `riot requirements <hash>` command, which uses pip-compile to generate a requirements lockfile for venv identified by the hash
- Adds the `-c/--recompile-requirements` flag to `riot run`. When set, this flag causes the requirements.txt lockfile and the venv itself to be regenerated. When omitted, the requirements lockfile is only built if it does not already exist.

Deprecation Notes
- Drop support for python 3.6

Bug Fixes
- Prefix paths are now capped to avoid generating paths longer than the FS can support.

New Features

- add `riot list --hash-only` option to list only venv short hashes