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.

roundup.anypy package - Python version compatibility layer
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Roundup currently supports Python 2.3 to 2.6; however, some modules
have been introduced, while others have been deprecated.  The modules
in this package provide the functionalities which are used by Roundup

- adapting the most recent Python usage
- using new built-in functionality
- avoiding deprecation warnings

Use the modules in this package to preserve Roundup's compatibility.

# vim: si