Add support for Python 3.6-3.9, drop EOL 2.6 and 3.2-3.4

[hugovk]

Fixes #28.

This PR:

Declares support for and tests new Python 3.6-3.8.

It tests on Python 3.9-dev, due out in October, but already in alpha. It fixes a removed deprecation to use collections.abc where available.

• Fedora are already creating early 3.9 builds, here's the babelfish bug they found: https://bugzilla.redhat.com/show_bug.cgi?id=1792057

This also drops support for EOL Python 2.6, 3.2-3.4.

• Python 2.6, 3.2 and 3.3 have been dropped by Travis CI so are not easily testable.

Add python_requires metadata to help pip install for people on older Python versions.

I haven't removed EOL 2.7 yet, but would recommend doing so as well. Please let me know if you'd like that removing too, but I mainly recommend getting the 3.9 fix in as soon as possible.

Thank you!

[hugovk]

Passing CI build:

• https://travis-ci.org/hugovk/babelfish/builds/639552462

Here's a build before the 3.9 fix, showing the problem:

• https://travis-ci.org/hugovk/babelfish/builds/639550086