[BUG] Taking a pointer to a const memoryview loses the constness

[da-woods]

Describe the bug

It's possible to get a non-const pointer from a const memoryview with no warnings and no casts

Code to reproduce the behaviour:

def f(const double[::1] x):
    cdef double *xptr = &x[0]

Expected behaviour

Constness should be preserved.

I think we probably have to make this a warning rather than an error - there's almost certain code depending on it

OS

Linux

Python version

3.11.4

Cython version

3.0.0 ish

Additional context

scikit-learn/scikit-learn#27125

You do get a C warning (with GCC) but not a Cython warning