PsychPython: Dropping 2.x support, adding opt-in Limited API support, and cleaning up Python abstractions

[aforren1]

Hi Mario,

Here's the follow-up to https://github.com/Psychtoolbox-3/Psychtoolbox-3/pull/766 (only took three months!).

I've reorganized as you recommended, and a few other things:

• Dropped updating PsychRuntimeGetPsychtoolboxRoot for now, until it actually gets exercised by code
• Made the Limited-API-compatible wheel building opt-in, rather than on by default for Python versions > 3.6. The idea being that users building the package for their personal use generally wouldn't benefit from ABI compatibility, and so that alternative/faster code paths relying on features not available in the Limited API would work by default.

All the demos work fine on a local Ubuntu 20.04 machine + a Win10 virtual machine, and all platform+Python version combinations are lightly tested during the wheel-building process (see e.g. https://github.com/aforren1/ptb-wheels/actions/runs/2862919054).

[kleinerm]

Ok. But please make these changes quick. This is material for the upcoming 3.0.18 -> 3.0.19 transition, given it breaks backwards compat by removing Python 2. I will release that likely in September.

[kleinerm]

Mostly good, with some small comments. Wrt. the last fixup commit however, I'd like to have the stuff integrated into the previous commits. And no trace of that gpl code in the first place, not even as a commit that gets undone in an immediate followup commit.

[kleinerm]

In the interest of time, as this needs to go into an initial 3.0.19 release and I don't want that release to be held up, i've made the changes i requested myself, and some other minor fixes/cleanups. You can find it as pull request #234. I will merge it once the PTB 3.0.19 release is up.

I've only lightly tested on Ubuntu 20.04 LTS without limited api, ie. if it actually builds, nothing more.

Closing this one. Thanks!