Migrate from tox to nox

[ocelotl]

nox uses a Python file instead of a .ini file (like tox does with tox.ini). This makes it possible to run code, something that seems useful.

[aabmass]

@lzchen it looks like we tried out nox in OpenCensus, any idea why we abandoned it?

[lzchen]

@aabmass
I believe it was due to its slowness.