Ensure file closing

[nim65s]

Hi,

Running tests with PYTHONWARNINGS=always on a project that depend of mdmail, I got:

…/lib/python3.8/site-packages/mdmail/api.py:101: ResourceWarning: unclosed file <_io.TextIOWrapper name='…/lib/python3.8/site-packages/mdmail/default.css' mode='r' encoding='utf-8'>
  css = open(default_css, encoding='utf-8').read()

So here is a quickfix.

While here, I updated the python versions that Travis should test:

• added 3.6 to 3.8, as they work well
• removed 3.3, as Travis doesn't provide it anymore:

  Unable to download 3.3 archive. The archive may not exist. Please consider a different version.

• removed 3.4, as lxml doesn't support it anymore:

  This lxml version requires Python 2.7, 3.5 or later.

[nim65s]

Hello there :)

This is just a friendly reminder that it's been a year, for a trivial fix.

Best.

[nim65s]

This looks no longer maintained, I made a fork: https://github.com/nim65s/nmdmail / https://pypi.org/project/nmdmail/