Add support for Python 3.7

[hugovk]

Add support for Python 3.7:

• Test on Travis CI
  • Xenial is required for 3.7 on Travis
  • Add python_requires to help pip install the correct version for the running Python
  • Update Trove classifiers

Also:

• Remove redundant code for EOL Python 2.6
• Some tidy up of Travis CI config:
  • sudo no longer required
    • https://blog.travis-ci.com/2018-11-19-required-linux-infrastructure-migration
  • Drop the dot in py.test
    • https://twitter.com/pytestdotorg/status/753767547866972160
  • Cache downloaded pip files

AppVeyor config not touched, that will be fixed by PR #190.

[codecov]

Codecov Report

Merging #192 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #192   +/-   ##
=====================================
  Coverage       0%     0%           
=====================================
  Files           1      1           
  Lines           1      1           
=====================================
  Misses          1      1

[thomasrockhu]

Thanks @hugovk !