objstr: Implement "%(key)s" % {} formatting for strings and dicts.

Also, make sure that args to "*" format specifiers are bounds-checked
properly and don't lead for segfaults in case of mismatch.
Merge pull request #663 from Rosuav/floatpercentx

Remove tests that fail under CPython 3.5
Add PEP 393-flags to strings and stub usage.

The test suite all passes, but nothing has actually been changed.