Support for Python 3

[vasekch]

Hi, initially thank you for the great solution.
It would be great to implement this for Python 3 as well.

First issue when executed with Python 3 is BaseHTPServer -> http.server difference:

Traceback (most recent call last):
  File "./snow-grafana-proxy.py", line 9, in <module>
    from BaseHTTPServer import BaseHTTPRequestHandler,HTTPServer
ImportError: No module named 'BaseHTTPServer'

[cinek810]

This sounds like a good idea, however, it may take some time to get this implemented. I'll work on that and tag commits fixing parts of python3 support and adding CI.