windows exception on Python 2.7.1

[rbrewer123]

I ran gup from the tip of the github repo on Windows using Python 2.7.1. I got an exception that os.readfile() does not exist.

I was able to get it working with Python 3.7, which seems to have os.readfile() on Windows.

[timbertson]

Thanks for the report. Now's probably a good time to just depend on python3 and drop python2, it's surely widely installed.

[rbrewer123]

I think dropping python2 support is reasonable.