Add Python 3.10 to the README

Author: grantjenks

README.rst

@@ -77,8 +77,8 @@ Features
 - Thread-safe and process-safe
 - Supports multiple eviction policies (LRU and LFU included)
 - Keys support "tag" metadata and eviction
-- Developed on Python 3.9
-- Tested on CPython 3.6, 3.7, 3.8, 3.9
+- Developed on Python 3.10
+- Tested on CPython 3.6, 3.7, 3.8, 3.9, 3.10
 - Tested on Linux, Mac OS X, and Windows
 - Tested using GitHub Actions