PYTHON-1951 Cap python 2 dnspython version at 1.16.x
PYTHON-1951 Cap python 2 dnspython version at 1.16.x

mongodb/mongo-python-driver,ShaneHarvey/mongo-python-driver,ShaneHarvey/mongo-python-driver,ShaneHarvey/mongo-python-driver,mongodb/mongo-python-driver,mongodb/mongo-python-driver