Dissallow Python 2.4, 2.5, 3.0, and 3.1
Remove obsolete utility code.
More obsolete code removal.
Check for Python 2.6+
More cleanup.
Restore some over-eager 3.1 removals.
Remove another py_set utility code reference.
uncomment vector assign function template
Merge pull request #275 from cython/no-old-python

Get rid of obsolete Python cruft.