Get rid of obsolete Python cruft

* Remove macros __Pyx_{NAME|DOC}STR
* Remove macros __Pyx_{Get|Set|Del}AttrString
* Remove macro  __Pyx_PyIndex_Check
* Remove workaround for missing BaseException in builtins
Remove conditional compilation handling old Python 3.0
Remove conditional compilation handling old Python < 2.6
Merge pull request #319 from cython/dalcinl/no-old-python

Get rid of obsolete Python cruft