Upgrade grpcio version to 1.39.0

[etherops]

grpcio version 1.30.0 has been problematic in 2 fashions.

1 - Compilation issues on later versions of macOS

As detailed in this github issue, grpcio is not building on some versions of Catalina and Big Sur. There are workaround (we used the following), but grpcio has fixed this in 1.39.0 (currently in pre-release).

brew update
brew install openssl
export LDFLAGS="-L/usr/local/opt/openssl@1.1/lib"
export CPPFLAGS="-I/usr/local/opt/openssl@1.1/include"
GRPC_PYTHON_BUILD_SYSTEM_ZLIB=1 GRPC_PYTHON_BUILD_SYSTEM_OPENSSL=1 pip install temporal-python-sdk

2 - Pre-build wheel unavailable for newer python versions

As detailed here pip install can take an inordinant amount of time to complete when a prebuilt wheel cannot be found for the platform. For grpcio==1.30.0 there is no wheel for python 3.9 because it was not available at the time. Upgrading to at least 1.34.0 will cover python 3.9 (according to https://pypi.org/project/grpcio-tools/1.34.0/#files).

Thank you!

[etherops]

Just noticed this PR: #17. Presumably 1.38 worked without code changes. 1.39 should also be as simple then.