Missing minimum grpcio version in requirements

[jbemmel]

pygnmi/pygnmi/client.py

Line 346 in c96b7ff

except grpc._channel._InactiveRpcError as err:

This change was introduced here: grpc/grpc@8c7b774

Version 1.33.1

[ I had an environment with grpcio version 1.18, and it crashed because this class didn't exist ]

[akarneliuk]

Hey @jbemmel ,

Thanks for pointing up. Would you like to submit PR? ;-)

Best,
Anton