Add new Sphinx Domain classifier

As seen here: https://pypi.org/search/?c=Framework+%3A%3A+Sphinx+%3A%3A+Domain

Author: jdillard

setup.py

@@ -43,6 +43,7 @@
     classifiers=[
         "Environment :: Console",
         "Environment :: Web Environment",
+        "Framework :: Sphinx :: Domain",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: BSD License",
         "Operating System :: OS Independent",