Conda cannot parse our requirements-test.txt file

[AlexWaygood]

This came up at a mentored sprint event at pycon just now -- a new contributor was trying to use a conda virtual environment instead of the stdlib venv, but conda does not seem to be able to parse our requirements.txt file because of this line:

https://github.com/python/typeshed/blob/c6b3211afaa8fa450e79a9c9674bbfd472593702/requirements-tests.txt#L2