feat: add support for Python 3.10 (#284)

Fix new deprecation warning for 'threading.Condition.notifyAll'.
diff --git a/setup.py b/setup.py
index 48c8979..d150bc0 100644
--- a/setup.py
+++ b/setup.py
@@ -87,6 +87,7 @@
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
+        "Programming Language :: Python :: 3.10",
         "Operating System :: OS Independent",
         "Topic :: Internet",
     ],