setup.py: advertise 3.11 support

setup.py

@@ -126,6 +126,7 @@ setup(name='dulwich',
           'Programming Language :: Python :: 3.8',
           'Programming Language :: Python :: 3.9',
           'Programming Language :: Python :: 3.10',
+          'Programming Language :: Python :: 3.11',
           'Programming Language :: Python :: Implementation :: CPython',
           'Programming Language :: Python :: Implementation :: PyPy',
           'Operating System :: POSIX',