This commit removes Six as a dependency for Pelican, replacing the
relevant aliases with the proper Python 3 imports. It also removes
references to Python 2 logic that did not require Six.
The default configuration for the warnings module changed some time ago
so we lost the ability to detect deprecation warnings and such early.
This commit sets up the test suite to redirect all warnings through our
logging system. Additionally we enable DeprecationWarnings as a default
and throw exceptions on all warnings related to pelican modules.
This enables output of warnings related to dependencies, while letting
tests only fail if the warnings are originating from pelican's source.
Also adding a test to detect if warnings cause an Exception as expected.
