Notable changes:
- Eliminate grpcio package dependency on protobuf package
- A new grpc.Channel.close method is introduced and correct use of
gRPC Python now requires that channels be closed after use.
- Python errors have become more verbose.
- GRPC_ENABLE_FORK_SUPPORT=false is no longer required when running
fork-exec.
- Relying on an explicit with statement or explicitly calling
Channel.close to release the underlying resources in Channel objects
is now a required coding practice starting in v1.13.0.
