Remove devel/py-ctypes (only needed by and supporting python24).
Remove PYTHON_VERSIONS_ACCEPTED and PYTHON_VERSIONS_INCOMPATIBLE
lines that just mirror defaults now.
Miscellaneous cleanup while editing all these files.
* Still don't depend on devel/mercurial *at all* (pending mercurial fixes).
Users will have to install devel/mercurial manually before cvs20hg
will run properly.