Add "check_size ignore" to all NumPy.pxd class declarations to silence the...

Add "check_size ignore" to all NumPy.pxd class declarations to silence the useless size warnings about them.
See https://github.com/numpy/numpy/pull/432#issuecomment-689846159
Also remove the useless "extern" modifiers from cdef classes declared in "extern" sections.

Cython/Includes/numpy/__init__.pxd

@@ -226,11 +226,11 @@ cdef extern from "numpy/arrayobject.h":
         # this field via the inline helper method PyDataType_SHAPE.
         cdef PyArray_ArrayDescr* subarray
 
-    ctypedef extern class numpy.flatiter [object PyArrayIterObject]:
+    ctypedef class numpy.flatiter [object PyArrayIterObject, check_size ignore]:
         # Use through macros
         pass
 
-    ctypedef extern class numpy.broadcast [object PyArrayMultiIterObject]:
+    ctypedef class numpy.broadcast [object PyArrayMultiIterObject, check_size ignore]:
         # Use through macros
         pass
 
@@ -914,7 +914,7 @@ cdef extern from "numpy/ufuncobject.h":
 
     ctypedef void (*PyUFuncGenericFunction) (char **, npy_intp *, npy_intp *, void *)
 
-    ctypedef extern class numpy.ufunc [object PyUFuncObject]:
+    ctypedef class numpy.ufunc [object PyUFuncObject, check_size ignore]:
         cdef:
             int nin, nout, nargs
             int identity