gen_protos.py should depend on the same grpcio dependency as setup.yp

The following line:
https://github.com/apache/beam/blob/3c81b41ff53ab890fd0c4d31d6fa1b157d9c0d81/sdks/python/gen_protos.py#L36

is a duplicate of what we have in setup.py. Instead of keeping them in syncing we should have a single source of truth.

Imported from Jira [BEAM-3375](https://issues.apache.org/jira/browse/BEAM-3375). Original Jira may contain additional context.
Reported by: altay.

Compare the grpcio dependency declaration at sdks/python/gen_protos.py#L36 with the corresponding entry in sdks/python/setup.py. Determine which existing dependency definition can serve as the single source of truth, then verify that both proto generation and Python packaging still obtain the required dependency consistently.

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