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

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.