Source install of the Python package breaks

[lepistone]

To reproduce, run for example docker run -it --rm python pip install --no-binary :all: opencensus-proto. The error will contain ModuleNotFoundError: No module named 'version' originating from https://github.com/census-instrumentation/opencensus-proto/blob/master/gen-python/setup.py#L17.

The wheel install works, at least with that architecture. However the source package is important:

• for architectures that don't have a wheel available
• in scenarios where wheels are intentionally disabled, for example when building Homebrew packages.

Thank you!