refactor: move RateLimitExceeded to roundup.cgi.exceptions RateLimitExceeded is an HTTP exception that raises code 429. Move it to roundup.cgi.exceptions where all the other exceptions that result in http status codes are located. Also make it inherit from HTTPException since it is one. Also add docstrings for all HTTP exceptions and order HTTPExceptions by status code. BREAKING CHANGE: if somebody is importing RateLimitExceeded they will need to change their import. I consider it unlikely anybody is using RateLimitExceeded. Detectors and extensions are unlikely to raise RateLimitExceeded. So I am leaving it out of the upgrading doc. Just doc in change log.

--- a/CHANGES.txt	Sun Aug 10 20:57:57 2025 -0400
+++ b/CHANGES.txt	Sun Aug 10 21:27:06 2025 -0400
@@ -13,6 +13,19 @@
 python 3.6 or newer (3.4/3.5 might work, but they are not tested).
 Roundup 2.4.0 is the last release to support Python 2.
 
+2026-XX-XX 2.6.0
+
+Fixed:
+
+- performance improvement to session_dbm.py:clean(). Also add warning
+  log message if clean takes longer than 3 seconds. (John Rouillard)
+- move RateLimitExceeded exception from roundup.exceptions to
+  roundup.cgi.exceptions. Also it now inherits from HTTPException
+  rather than Exception since it is an HTTP exception. (John
+  Rouillard)
+
+Features:
+
 2025-07-13 2.5.0
 
 Fixed: