Remove environs

[lucyb]

See Dave's comment in this PR.

Airlock specifically didn't use environs because its transitive dependencies seemed pretty heavyweight for the value it gives. We're only using it in settings.py, so it seems unlikely we're getting the full value from it and that we can easily remove it.

[StevenMaude]

The solution used in Airlock is to rely on just importing the .env and directly calling out to os.environ(), with a helpful error message if an environment variable is missing telling you to import via your shell.

See https://github.com/opensafely-core/airlock/blob/6148179ac2b0c669b0388fbaf2ce31ddeeaa3324/airlock/settings.py