Remove official support for PySide

[ccordoba12]

This removes references to PySide in our official docs, in our Readme and in setup.py.

However, there is (and will always be) support for PySide in bootstrap.py and in our Qt shim, in case people want to keep using it.

Reasons to do this:

1. PySide future is uncertain and very little is happening with it currently.
2. Now that we've moved to support PyQt5 officially (see PR Use PyQt5 as default API #2879), I want to reduce the number of bindings we support so it be easier to maintain Spyder.
3. PySide support is currently broken in master (Preferences is crashing with it)
4. Nobody has helped to support it (besides Pierre and me).

This was not only my decision, all @spyder-ide/core-developers agreed with it.

[ccordoba12]

Letting @picca know about this one.

[techtonik]

-1 on that.

[techtonik]

This was not only my decision, all @spyder-ide/core-developers agreed with it.

This link is 404.

[techtonik]

Please link to actual discussion.

[ccordoba12]

@techtonik, we discussed it in our internal gitter chat, and during a couple of hangouts we had some months ago. So there's no public record of it, sorry.

As I said, Spyder will continue to work with PySide, so there's no problem with that. We only stopped to support it officially, meaning that we (the dev team) are not going to solve more issues related to it.

Support for it will have to come from the community :-)