Context

https://motor.readthedocs.io/en/stable/developer-guide.html#synchro

The section in the developer guide that discusses Synchro instructs one to run the tests with a tox env/target that is out-of-date. It suggests. "Use tox -e synchro37 to check out PyMongo’s test suite and run it with Synchro."

The things is, synchro37 no longer exists. It's just testenv:synchro.

Definition of done

Change one line in docs. Check that it builds. Update changelog (maybe?).