[BUG] values used for exception_value are really inconsistent

Describe the bug

This is something I'm running into while trying to sort out implicit exception_value for fused types. But it's a general problem not related to fused functions.

Essentially for floating point types, Cython uses a number of different representations of the exception value, and this leads to mismatches where there should be no mismatch

Code to reproduce the behaviour:

cdef double return_double(double arg, bint fail):
    if fail:
        raise RuntimeError
    return arg

ctypedef double (*func_ptr1)(double, bint) except? -1

def test():
    # Test that we can assign to the function pointer we expect
    cdef func_ptr1 p1 = return_double

This fails with

Error compiling Cython file:
------------------------------------------------------------
...

ctypedef double (*func_ptr1)(double, bint) except? -1

def test():
    # Test that we can assign to the function pointer we expect
    cdef func_ptr1 p1 = return_double
                        ^
------------------------------------------------------------

fused_rettype3.pyx:10:24: Cannot assign type 'double (double, bool) except? -1' to 'func_ptr1'. Exception values are incompatible.

The reason is that the type for return_double has an exception_value of '-1' (note a str). The type for func_ptr1 has an exception_value of '-1.0' (note also a str).

Therefore they're "incompatible" and the assignment fails.

Expected behaviour

No response

OS

No response

Python version

No response

Cython version

Current master

Additional context

I think the mistake is using strings to represent exception_value

cython / cython