The 4.0 transaction spec says:

startTransaction SHOULD report an error if the driver can detect that transactions are not supported by the deployment. A deployment does not support transactions when the deployment does not support sessions, or maxWireVersion < 7, or the topology type is Sharded

https://github.com/mongodb/specifications/blob/cf7248b2574b1a5397a6e18362acfe66f2b55a43/source/transactions/transactions.rst#starttransaction

However, pymongo never implemented this check. When 3.7.2 runs a transaction against MongoDB 4.2, it will get an error like this on the first command that goes to a new mongos:

{
  'ok': 0.0,
  'errmsg': 'cannot continue txnId -1 for session ... with txnId 1',
  'code': 251,
  'codeName': 'NoSuchTransaction',
  'errorLabels': ['TransientTransactionError']
}