Update code for newer NumPy

1. Removed use of vendored numpy.pxd, replaced it with cimport numpy
   to resolve runtime warning during import of change in size of
   broadcast struct
2. Use explicit namespace for NumPy C-API symbols imported by
   Cython. This caught several issues where C-API ndarray object
   was used where Python object of type np.ndarray was intended.
3. Resolve compilation warnings reported by Clang about ignored
   type qualifier during casting.

Local build now passes, import does not report the runtime warning,
and test suite passes.

Author: oleksandr-pavlyk