Add classifier for python 3.10

This is so the version shield ( [![PyPI](https://img.shields.io/pypi/pyversions/rdflib.svg)](https://pypi.python.org/pypi/rdflib) ) shows up correctly in future releases.

Author: aucampia

setup.py

@@ -71,6 +71,7 @@ def open_local(paths, mode="r", encoding="utf8"):
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
         "License :: OSI Approved :: BSD License",
         "Topic :: Software Development :: Libraries :: Python Modules",
         "Operating System :: OS Independent",