Port to Python 3

This patch series ports gracedb-client to Python 3, and enables Debian packaging for Python 2 and 3. Here is evidence of the correctness of the port:

• I was able to run BAYESTAR (command: bayestar_localize_lvalert -N G211117) which exercises several different use cases of gracedb.

One caveat is that the m2crypto and cjson dependencies do not support Python 3. We can do without cjson if we use the json module from the Python standard library. The lack of m2crypto likely means that only basic auth will work with Python 3.