Drop some compat for python < 2.6

[dlax]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Description:

Per changelog, support for python 2.4 and 2.5 got dropped in version 1.2.0 but there remained some compat code, some of which is here dropped. One step towards #394.

[coveralls]

Coverage increased (+0.4%) to 77.832% when pulling 5edca66 on dlax:drop-old-py2-compat into c93aeb2 on urwid:master.

[coveralls]

Coverage increased (+0.01%) to 77.898% when pulling f2b927e on dlax:drop-old-py2-compat into 2cc5489 on urwid:master.

[ulidtko]

Nice! LGTM.

Sorry it took so long to review this, that a conflict appeared (ironically, with another py2/3 compatibility-related commit).

It's a simple 1-line conflict in imports though. I've solved it using GitHub web UI. Merging as soon as tests pass.

[ulidtko]

Hmmm... GitHub doesn't show it for some reason, but there're bytes-related test failures on Py 3.5 & 3.6 — see Travis CI https://travis-ci.org/github/urwid/urwid/builds/763275814

[dlax]

The failure comes from master; see, e.g., https://travis-ci.org/github/urwid/urwid/builds/758944725