Cython 0.29 broke support for C++ enum classes

Before https://github.com/cython/cython/commit/be4ca1d685e8d703f2285183147f8587da56fe6d (a fix for #2186), on this input:

cpdef enum E "E":
    A

Cython was generating this:

/* CIntToPy */
static CYTHON_INLINE PyObject* __Pyx_PyInt_From_enum__E(enum E value) {
    const enum E neg_one = (enum E) -1, const_zero = (enum E) 0;

Since https://github.com/cython/cython/commit/6079a5b0b0936feb2ec339a1f31d7d708ae6d3a3 it is generating this:

/* CIntToPy */
static CYTHON_INLINE PyObject* __Pyx_PyInt_From_enum__E(enum E value) {
    const enum E neg_one = (enum E) ((enum E) 0 - (enum E) 1), const_zero = (enum E) 0;

This works for C enums, but fails to compile for C++ enum classes (https://godbolt.org/z/NY8PJT):

enum class E { X };
const enum E neg_one_class_28 = (enum E) -1;
const enum E neg_one_class_29 = (enum E) ((enum E) 0 - (enum E) 1);

<source>:3:54: error: invalid operands to binary expression ('enum E' and 'enum E')
const enum E neg_one_class_29 = (enum E) ((enum E) 0 - (enum E) 1);
                                          ~~~~~~~~~~ ^ ~~~~~~~~~~

[orivej]

They are not fully supported: for example, it seems difficult to use enum class variables as function arguments; but what Cython already has is enough to wrap them. The original example in #1603 can be encoded like this:

cdef extern from "headername.h" namespace "lol::rofl":
    cdef enum rofl:
        SOMETHING
        MOAR

cdef void f(int arg):
    print(arg)

f(<int>rofl.MOAR)

[cf-natali]

https://github.com/cython/cython/pull/3803 fixes this (tested locally).