scanned stream is unicode, so use unicode strings for comparisons to avoid redundant recoding in Py2

--HG--
extra : rebase_source : d9445f29b6d417ef02c53f7f9b490843de81501c

__Pyx_PyObject_IsTrue now has only a single branch, even at -O0.
(The bitwise | is intentional.) This is only a 2.5% or so speedup
in my microbenchmarks, but it's something.

safety fix: a string as source input to the scanner should always be considered a .pyx file for now, regardless of how you name it