gh-106320: Remove private _PyLong_New() function

[vstinner]

Move the following private API to the internal C API (pycore_long.h):

• _PyLong_Copy()
• _PyLong_FromDigits()
• _PyLong_New()

No longer export most of these functions.

• Issue: C API: Remove private C API functions (move them to the internal C API) #106320

[skirpichev]

@vstinner, we use _PyLong_New() in the gmpy2, e.g. in the GMPy_PyLong_From_MPZ() method. What's now the intended replacement for this private API?

[skirpichev]

Another user is the sage:

$ git grep _PyLong_N
src/sage/libs/gmp/pylong.pyx:from cpython.longintrepr cimport _PyLong_New, py_long, digit, PyLong_SHIFT
src/sage/libs/gmp/pylong.pyx:    L = _PyLong_New(pylong_size)

[vstinner]

we use _PyLong_New() in the gmpy2, e.g. in the GMPy_PyLong_From_MPZ() method. What's now the intended replacement for this private API?

Would you mind to open a separated issue describing how you use _PyLong_New() and which API do you need? _PyLong_New() changed a log in Python 3.12: see also issue #111140.

[skirpichev]

I've opened #111415