Include version.py in distribution for certain packages

[lzchen]

Applies to:

opencensus-context
opencensus-ext-flask
opencensus-ext-requests

specifically requested by a user.

[aabmass]

I think `context/opencensus-context/opencensus/common/version.py may conflict with this package https://github.com/census-instrumentation/opencensus-python/blob/master/opencensus/common/version/__init__.py?

Wouldn't they existin the same namespace package?

[lzchen]

@aabmass

I think `context/opencensus-context/opencensus/common/version.py may conflict with this package https://github.com/census-instrumentation/opencensus-python/blob/master/opencensus/common/version/__init__.py?

I moved context/opencensus-context/opencensus/common/version.py to context/opencensus-context/opencensus/common/runtime_context/version.py. This way it would be more clear that the version.py belongs to the context package when going through the namespace packages.