Python 2 cleanups: tests/branching/imports/misc

[aktech]

I have tried to remove all traces of Python 2 in this PR. Fixes #240

• [x] remove .travis.yml as it's not used anymore
• [x] remove __ future__ imports
• [x] remove PY2 branch code
• [x] remove check for PY3
• [x] remove Python 2 only tests
• [x] remove TOML fallback
• [x] Misc

[dshivashankar1994]

Few of more minor comments:

1. We can remove pyflyby._log._NoRegisterLoggerHandlerInHandlerListCtx is no longer required
2. Condition - https://github.com/deshaw/pyflyby/blob/master/lib/python/pyflyby/_py.py#L404 can be removed too
3. pytest.ini won't require - IGNORE_EXCEPTION_DETAIL (?)
4. There are several doctest which we are skipping. We can remove them if specific to python 2 related. Ex: ex1, ex2, ex3
5. Few test funcs like ex1,

[Carreau]

I suggest we get that in, and iterate if there are remaining python-2 branches.

[aktech]

I suggest we get that in, and iterate if there are remaining python-2 branches.

Thanks! I have handled most things. I'll get this in now, will add anything remaining in a follow up PR.