Enforce the use of Pymongo 3.12.2 in our Evergreen tests

The Ubuntu 20.04 Performance tests in evergreen fail as the uploading of the document to Atlas is unsuccessful.

After the perf tests are run, upload_stats_atlas.py script uploads the detailed output in JSON format to Atlas. The script calls the database_names() function which has been deprecated in favour of list_database_names() starting pymongo 4.0

Using pymongo 4.0 also results in the many-collection-test in Evergreen failing.