Add 1.26.10 release to CHANGES.rst
+1.26.10 (2022-07-07)
+====================
+
+* Removed support for Python 3.5
+* Fixed an issue where a ``ProxyError`` recommending configuring the proxy as HTTP
+ instead of HTTPS could appear even when an HTTPS proxy wasn't configured.
+
1.26.9 (2022-03-16)
===================
