Preliminary support for Python 3.14a2.

[icemac]

I hacked something together so the tests do not fail for Python 3.14.
But I am completly unsure if this is the way we want/should go.

Fixes #334.

[dataflake]

I can see the fix for that missing __classdictcell__ slot, but what's the IByteString change for?

Also, if you only want to test "is this Python 3.14", why do you need all those definitions for the older versions?

[d-maurer]

Why do you remove the IByteString definitions (in common.builtins and common.collections) for Python 3.14? The interface is likely useful for that version as well.