Add Python 3 compatibility

[lelit]

This set of commits do tiny cleanups and then make the library usable under Python 3, at least as far as automatic tests can tell.

[markfink]

wow. that is really cool! I was planning to add python3 support myself but did not find the time yet.
whaaaaat is pretty new. are you using it in your projects where you need python3 support? I can not look into this pull request today but I promise do do so asap.

[lelit]

Not yet, but if/when this is accepted, I will try to switch my tinject tool from inquirer to wha+t, should not be hard.

[markfink]

@lelit a fine piece of work. thank you so much!

[lelit]

You're welcome! Unfortunately, it seems that I didn't spot all the places: common.py needs the

import sys
...
PY3 = sys.version_info[0] >= 3

if PY3:
    basestring = str

snippet too.

[lelit]

I may submit another PR for this if you prefer, but I think the best would be to create a test covering setup_validator and setup_simple_validator too. Let me know and I will try to do that as well.

[markfink]

you are absolutely right, the test coverage is not where I want it to be. If you like to contribute the test please go ahead.
please be aware that ptk 2.0 is coming: prompt-toolkit/python-prompt-toolkit#446
this plays into our direction by providing dialogs but might also cause some rework in whaaaaat