Currently, running any setuptools commands using `setup.py` generates a
warning like so:
        warning: manifest_maker: MANIFEST.in, line 21: unknown action 'dev-requirements.txt'

The syntax is invalid and got accidentally introduced in !637 but it's a
shame that this is not a warning and no obvious way to make it an error
if there are invalid things in MANIFEST.in file.

MANIFEST.in: Fix include for dev-requirements.txt

See merge request !704

When fixing terminal notifications, I had introduced a bug
with accesses to `os.environ` which triggered KeyError, this
patch fixes it.

Valentin David authored Aug 22, 2018 and Tristan Van Berkom committed Aug 23, 2018

Fixes #576.