Allow installing git blobs without cloning

[larsoner]

• Make sure to include reasonable tests for your change if necessary

• We use towncrier for changelog management, so please add a news file into the changelog folder

I don't think this is really changelog-worthy, but happy to add one if a maintainer thinks it is! Same with a test...

Allows installing

pip install https://github.com/pytest-dev/pytest-xdist/archive/refs/heads/master.zip

directly and obtaining a correct version from scm. Nicer than git+ because you don't have to clone the whole repo for it to work.

I've been using this for a year or two in MNE-Python and other repos without issues.