Drop support for EOL Python 3.4

[hugovk]

Python 3.4 is EOL since 2019-03-16 and no longer receiving security updates (or any updates) from the core Python team.

This allows 5 jobs to be removed from the build (3 Travis, 2 AppVeyor), and allows the project to concentrate on supported Python versions.

This PR also adds python_requires to help pip install the correct package version for their running Python.

[codecov-io]

Codecov Report

Merging #421 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #421   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files          50       50           
  Lines        6952     6952           
  Branches     1328     1328           
=======================================
  Hits         6312     6312           
  Misses        481      481           
  Partials      159      159

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[gsnedders]

pypa/pip@2c1010e dropped support for 3.4 in pip, shipped in 19.2.