Python 3 version

[marcverhagen]

There is no hurry with this, but at least I want to start making the current code incrementally more Python 3 compliant.

In the first stage I want to do only those things that require no extra installation for TTK users, no matter how simple (so, for example, not even 'pip install future', which would be needed so you can use 'import future').

A few notes on doing this:

• use python-modernize to give hints on the idioms to change
• use pylint --py3k for more hints on Python3 compatibility
• don't bother using a separate branch, this can be done incrementally whenever working on a module, so the develop branch or any issue or feature branch is fine

Note that issue #22 is part of this.

Also keep an eye on issue #74 while doing this.

[marcverhagen]

Finally started this. Basically following the steps in https://portingguide.readthedocs.io/en/latest/process.html and http://python3porting.com/.

I am dropping the initial requirement that no extra installations are allowed since I am not trying to get a 2.7 version for users to work with without extra effort.

Notes on this are written to /docs/notes/python3.md.

[marcverhagen]

Code now runs on Python 3.8.5 and passes all tests. Minor issues probably remain with peripheral code