We should have 2 evergreen tasks - 1 that runs synchrotest with PyMongo master and the other that runs it with the pinned PyMongo release.

This might require more work than is apparent initially as synchrotest requires installation of pymongo from sources which might be somewhat difficult to automate for the pinned version. Ideally, the solution would not require manually updating the pymongo version in evergreen/tox configuration after each Motor release but this might turn out to be the only viable solution.