python3-Cython: fix memory leak + debug output for generators

[tornaria]

cython/cython#5690
cython/cython#5725

Both are merged in cython 3.0.3, however that release breaks scipy.

Testing the changes

• I tested the changes in this PR: briefly

The memory leak was detected due to 3 doctest failures in sagemath (when built with cython 3) and this PR fixes it.

The debug output issue symptom is that sagemath can't be built unless one sets SAGE_DEBUG=no (however, this only triggers when python3-lxml is installed).

cc: @ahesford

[tornaria]

Alternatively, we could try updating to 3.0.3 + cython/cython#5752 which claims to fix the scipy build regression. I haven't tested that one though.

Edit: or just wait for 3.0.4. This only affects sagemath with cython 3 which is not what we ship. I already have patched cython in my development box. As long as this is fixed by the time sagemath 10.2 is released (cython 3 only), we'll be fine.

[ahesford]

Thanks for digging into this. Because you know this fixes issues with sagemath, let's take these patches for now and wait to see if Cython 3.0.4 fixes SciPy builds.