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Another revision of the patch, now quite close to PEP 461 as proposed. Changes from PEP 461:
- include %a
- add -2 command-line flag. When enabled have %s fallback to calling PyObject_Str() and encoding to ASCII and also enable %r as alias for %a.
Changes from previous patch:
- remove __ascii__ special method, %s will only accept objects that
 implement __bytes__ or the buffer API, unless -2 command line is used
- use buffer API if available
- add -2 command-line option
- Add prototypes for PyBytes_Format and _PyUnicode_FormatLong
- improve some exception messages
Reference counting in PyBytes_Format is quite hairy, could use some review. The code is nearly the same as Python 2.x stringobject.c.