Change the PY3 detect way for more compatible

I'm using python 2.7.
I have a file named `http.py`

When start my program, `urllib3.connection` throw an Error,
that's because `urllib3.connection` try import `http.client` module directly.

By this way to detect the python version, and do other imports.
(`http.client`  is a py3 standard library)


So, I make this changes.  using `packages.six.PY3` instead of try import directly.