feat: add support for Python 3.10 (#882)

diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 0e784af..255f33c 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -19,7 +19,7 @@
   using ``nox -s docgen``.
 
 - The change must work fully on the following CPython versions:
-  3.6, 3.7, 3.8, 3.9 across macOS, Linux, and Windows.
+  3.6, 3.7, 3.8, 3.9, 3.10 across macOS, Linux, and Windows.
 
 - The codebase *must* have 100% test statement coverage after each commit.
   You can test coverage via ``nox -e cover``.