You can't rely on tox alone to test the various versions of python
because they might not be installed. Instead specify a build matrix by
hand. This also allows for a cleaner after_script for coveralls.

This correctly parses options of the form "E302,W405", which before this
was split into individual characters. This resulted in all errors and/or
being considered when ignoring/selecting.

Since specifying just "E" disables all errors, having a list of
characters meant that it always encountered an individual "E" and thus
disabled all errors.

Python 3.7 will be released in about 2 months.[1] It also introduces
`async` as a reserved keyword which is currently used by pylama as a
module, variable, and option name.

This also removes python 3.3 from `.travis.yml`.  Python 3.3 reached
its end of life at 2017-09-29.[2] Without `py33` specified in
`tox.ini` the tests fail anyway.

[1] https://www.python.org/dev/peps/pep-0537/#schedule
[2] https://www.python.org/dev/peps/pep-0398/#x-end-of-life

Fixes W606 'async' and 'await' are reserved keywords starting with Python 3.7

This continues to accept `--async` as an argument and `async = 1` in
config files.  Therefore this commit should not be breaking.