`mypy.codec` and `typing` are not installable for python2

[o11c]

Working on #915, when mypy is installed rather than run from the current dirrectory, the mypy.codec tests only work in python3 mode because there is no setup.py for python2.

Some previous discussion on #732 and #724.

[o11c]

Current thought: move it to mypy_codec package instead of a subpackage.

In the mean time I am XFAILing it so I can work on the rest of the bug.

[o11c]

Alternatively: move current setup.py to setup3.py, add a setup2.py, and have setup.py dispatch automatically.

[o11c]

Also affects typing, had to do some nasty hacks to make testpythoneval work for now.

[JukkaL]

I'd rather keep the existing setup.py unmodified and add a separate package mypy-encoding (with a dash instead of an underscore) that supports both Python 2 and Python 3. The official term is Source File Encoding: https://www.python.org/dev/peps/pep-0263/

For typing we need a separate package as well. At some point the idea was to have typing in https://github.com/JukkaL/typing but I haven't that in a while. We'll have to figure this out.

[JukkaL]

Actually, this is multiple independent issues. I'm going to split this into two separate issues.

[JukkaL]

This is now an umbrella task for #919 and #920. I'm keeping this open for now, but further discussion should probably happen under the subtasks.

[gvanrossum]

No, we should deprecate the codec and delete the code. See my comment in #919.