Change to source.python.3

Confirmed that:

1. When file syntax is set to the in-built "Python", Sublime uses the build-in Python build system (it uses the Python in your path).
2. When file syntax is set to "Python3", plugins that match on scope "source.python" continue to match, i.e., Anaconda works :).
3. Build selector is able to specifically match "source.python.3" and takes precedence over something that matches "source.python".