setup.py: add classifier on supported Python version

We're (hopefully) about to change the specifics about the Python
versions we support.  But, until then, let's be clear about the
Python version (2.7) that we support.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

setup.py

@@ -154,6 +154,7 @@ if __name__ == '__main__':
               "Operating System :: POSIX",
               "Topic :: Software Development :: Quality Assurance",
               "Topic :: Software Development :: Testing",
+              "Programming Language :: Python :: 2.7",
               ],
           packages=find_packages(exclude=('selftests*',)),
           data_files=get_data_files(),