Python 3.10 compatibility

[jamesjer]

The Fedora project is building all python packages with an alpha version of python 3.10, in an attempt to find and fix problems in advance of the python 3.10 release. The cypari2 package fails to build because Py_SIZE(x) is no longer an lvalue. A macro that sets the size appropriately by python version is available in Cython 0.29.20 and later: __Pyx_SET_SIZE(x).

[jamesjer]

For the record, the change to make Py_SIZE not an lvalue proved too disruptive. It has been reverted for now, so the current code should continue to work with python 3.10. This change is still correct, however, and will be needed eventually, just not as soon as I thought.

[kliem]

Sorry, I opened another pull request and while I thought I had seen something like this before, I couldn't find it (I was looking in issues only). It seems you provided exactly the fix that I was looking for and you solution is much simpler than mine.

[jamesjer]

No problem at all. I'm glad this approach is useful.

[videlec]

I don't like so much depending on Cython>=0.29.20. Ubuntu 20.04 (LTS) ships 0.29.14, see https://packages.ubuntu.com/focal/cython.

[kliem]

If this is the only problem, we can instead depend on cython 0.29: https://github.com/cython/cython/blob/0.29/Cython/Includes/cpython/longintrepr.pxd

and just copy the corresponding function as in my approach #115.

[kliem]

@videlec Can you please comment on how you wish to proceed.

[videlec]

Any approach that works on all Cython versions >= 0.29.14 would fit.

[kliem]

Included in #120.