Support pyxl

[JukkaL]

Pyxl uses a custom source file encoding. Somehow get it to work.

[o11c]

If the encoding is installed, shouldn't it Just Work™ since the parser operates directly on decoded unicode objects?

Or are there python2-specific issues with how parsing works? I admit I'm operating from a python3 perspective.

The "mess with builtins" is the nastier part IMO.

[o11c]

Or wait, is the issue "how to use both coding: mypy and coding: pyxl" ? In that case, I think it's possible to modify the lookup to do multi-stage encoding with coding:mypy-pyxl ...

[JelleZijlstra]

I'm guessing this is no longer an issue, but somebody at Dropbox should confirm.

[gvanrossum]

Oh man, we're really not very good at closing issues when they're fixed. This was fixed by https://github.com/gvanrossum/pyxl3 over two years ago.

[emmatyping]

@gvanrossum perhaps we should adopt @msullivan's proposal in #4849? That would certainly help :)