Add support for Python 3.7 and fix deprecation warnings

[hugovk]

• Includes Drop support for EOL Python 2.6 and 3.3 #173 so the CI can pass and to avoid merge conflicts.

• A little workaround is needed for 3.7 on Travis, see Add Python 3.7 option travis-ci/travis-ci#9815.

• Also fixes this deprecation warning on Python 3.7 (with pytest):

================================================= warnings summary =================================================
/usr/local/lib/python3.7/site-packages/_pytest/config/findpaths.py:42: RemovedInPytest4Warning: [pytest] section in setup.cfg files is deprecated, use [tool:pytest] instead.
  config=config,

/private/tmp/configobj/src/configobj/__init__.py:546: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  elif isinstance(value, collections.Mapping) and not unrepr:

-- Docs: https://docs.pytest.org/en/latest/warnings.html

• And fixes this deprecation warning in pytest (with Python 2.7 and 3.7):

================================================= warnings summary =================================================
/usr/local/lib/python3.7/site-packages/_pytest/config/findpaths.py:42: RemovedInPytest4Warning: [pytest] section in setup.cfg files is deprecated, use [tool:pytest] instead.
  config=config,

-- Docs: https://docs.pytest.org/en/latest/warnings.html

[coveralls]

Coverage increased (+0.04%) to 81.503% when pulling fb66a35 on hugovk:add-3.7 into 5b5de48 on DiffSK:master.

[coveralls]

Coverage increased (+0.04%) to 81.503% when pulling fb66a35 on hugovk:add-3.7 into 5b5de48 on DiffSK:master.

[coveralls]

Coverage increased (+0.04%) to 81.503% when pulling fb66a35 on hugovk:add-3.7 into 5b5de48 on DiffSK:master.

[coveralls]

Coverage increased (+0.04%) to 81.503% when pulling fb66a35 on hugovk:add-3.7 into 5b5de48 on DiffSK:master.

[jhermann]

Thanks.

[hugovk]

You're welcome!