When executing a retryable read with an implicit session, pymongo uses one implicit session for the initial attempt and another implicit session for the retry.

This is a bug as the spec says:

The minimum server version is 3.6 because

• It gives us version parity with retryable writes.
• It forces the *retry attempt to use the same implicit session, which would make it it easier to track operations and kill any errant longer running operation*.
  ...

This was uncovered by the tests added in PYTHON-2075.