Start making f2py PyPy compatible

- numpy/f2py/f90mod_rules.py: Replace "np.intp().itemsize" by
  "np.dtype('intp').itemsize", since the latter is supported by
  PyPy.
- numpy/f2py/src/fortranobject.c: check_and_fix_dimensions():
  Refactor using PyArray_NDIM() and PyArray_DIM() instead of
  direct field access.

Tested by recompiling and testing CPython numpy. Works.