Fixes https://github.com/cython/cython/issues/1071

There's now a fairly wide range of valid syntax for declaring things in pyx files and it should all be supported when cimporting from them.

This reverts commit a333d6a8.

The change did not resolve the test issue in travis.

Closes https://github.com/cython/cython/issues/3775

This is a follow up to https://github.com/cython/cython/pull/3307

Closes https://github.com/cython/cython/pull/3763

Use NumPy 1.18.x for testing on travis as long as 1.19.[01+] ships a numpy.pxd that is incompatible with Cython 3.0.

Try to fix NumPy test failures by not setting the "NPY_NO_DEPRECATED_API" #define for NumPy 1.19.[01].

Change "Py_intptr_t" declaration in numpy.pxd to what CPython uses as fallback. "int" seems overly lazy if it tends to be larger on many systems.