Use sizeof, not cython.sizeof

Removes importing cython as the Shapely runtime does not require it.
Pointed out by @pelson in #156.
Merge branch 'no_cython_import' of github.com:jwass/Shapely into jwass-no_cython_import

Conflicts:
	shapely/speedups/_speedups.pyx
Merge branch 'jwass-no_cython_import'