[#233] Cleanup testsuite

[elbenfreund]

In order to make the testsuite play nice with the current external libraries, the following changes were made:

• Change SafeConfigParser to ConfigParser
• Drop python2 support from the test suite (tox , travis)
• Use explicit kwargs instead of args when instantiating Gtk.Label/Button etc instances.
• Remove version pinning for docs.pip and tests.pip.

Closes: #233