Add generic mixin class metaclass to bridgedb.util.
Change bridgedb.test.util.TestCaseMixin to be a bridgedb.util.mixin.
Remove obsolete doctest running code in bridgedb.test.util.
Run `pylint --rcfile=.pylintrc --generate-rcfile > .pylintrc`.

This refreshes the pylint config file with new settings from upgrading to
version 1.1.0.
Add pip requirements.txt file for testsuite dependencies.

 * ADD new file, .test.requirements.txt, for specifying packages and
   their versions which are required for running BridgeDB's test suite.