Add 1.26.4 release to CHANGES in main
Changes
=======
+1.26.4 (2021-03-15)
+-------------------
+
+* Changed behavior of the default ``SSLContext`` when connecting to HTTPS proxy
+ during HTTPS requests. The default ``SSLContext`` now sets ``check_hostname=True``.
+
+
1.26.3 (2021-01-26)
-------------------
