Add additional PyPI trove classifiers

Since we only support Python 3 and only test against CPython (the
reference implementation) it would be helpful to indicate these facts
with our trove classifiers.

setup.py

@@ -74,11 +74,13 @@ def get_version(version_file):
         # Specify the Python versions you support here. In particular, ensure
         # that you indicate whether you support Python 2, Python 3 or both.
         "Programming Language :: Python :: 3",
+        "Programming Language :: Python :: 3 :: Only",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3.10",
+        "Programming Language :: Python :: Implementation :: CPython",
     ],
     python_requires=">=3.6",
     # What does your project relate to?