Exclude grpcio v1.30.0, which breaks Pulumi Python programs

[justinvp]

Works around #4881

I tested installing locally in a clean virtual environment:

$ venv/bin/pip install -e /Users/justin/go/src/github.com/pulumi/pulumi/sdk/python/env/src
Obtaining file:///Users/justin/go/src/github.com/pulumi/pulumi/sdk/python/env/src
Collecting protobuf>=3.6.0 (from pulumi==2.5.0a1592953913)
  Using cached https://files.pythonhosted.org/packages/12/b5/9347e30e11c040f5ca24f079d6f06485280c49b2a9f894b5400e27d4d6d1/protobuf-3.12.2-cp37-cp37m-macosx_10_9_x86_64.whl
Collecting dill>=0.3.0 (from pulumi==2.5.0a1592953913)
  Using cached https://files.pythonhosted.org/packages/e2/96/518a8ea959a734b70d2e95fef98bcbfdc7adad1c1e5f5dd9148c835205a5/dill-0.3.2.zip
Collecting grpcio!=1.30.0,>=1.9.1 (from pulumi==2.5.0a1592953913)
  Using cached https://files.pythonhosted.org/packages/59/75/5a949d048d96ecb3c304f14975f8a0965cb0d3c3f5f5f6b3f4e15c3d49eb/grpcio-1.29.0-cp37-cp37m-macosx_10_9_x86_64.whl
Requirement already satisfied: setuptools in ./venv/lib/python3.7/site-packages (from protobuf>=3.6.0->pulumi==2.5.0a1592953913) (41.2.0)
Collecting six>=1.9 (from protobuf>=3.6.0->pulumi==2.5.0a1592953913)
  Using cached https://files.pythonhosted.org/packages/ee/ff/48bde5c0f013094d729fe4b0316ba2a24774b3ff1c52d924a8a4cb04078a/six-1.15.0-py2.py3-none-any.whl
Installing collected packages: six, protobuf, dill, grpcio, pulumi
  Running setup.py install for dill ... done
  Running setup.py develop for pulumi
Successfully installed dill-0.3.2 grpcio-1.29.0 protobuf-3.12.2 pulumi six-1.15.0