Log Message:
py-mongo: updated to 3.7.2

Version 3.7.2 fixes a few issues discovered since the release of 3.7.1.
- Fixed a bug in retryable writes where a previous command's "txnNumber"
  field could be sent leading to incorrect results.
- Fixed a memory leak of a few bytes on some insert, update, or delete
  commands when running against MongoDB 3.6+.
- Fixed a bug that caused :meth:pymongo.collection.Collection.ensure_index
  to only cache a single index per database.
- Updated the documentation examples to use
  :meth:pymongo.collection.Collection.count_documents instead of
  :meth:pymongo.collection.Collection.count and
  :meth:pymongo.cursor.Cursor.count.