unable to build new py3 thrift changes in upstream

[vivsy]

Hi, we pulled in the new changes from upstream that included the addition of the py3 namespace to some of the thrift files as well as some python tests trying to use the generated python file- however, I don't think the file was generated on our end and thus the tests errored out.

I tried looking at the corresponding cpp files that fbthrift generated and noticed that there was a lot more explicit definition of those files in the Makefile.am but I didn't see the same for the python files. Wondering if there's anything that I missed or if we need to somehow manually generate the python files? Thanks!