Upgrade Cython to 0.29.21

[mkoeppe]

We upgrade Cython to 0.29.21

(​https://github.com/cython/cython/blob/master/CHANGES.rst#02921-2020-07-09)

This is supposed to fix many bugs compared to our current 0.29.17, but a bug reported with the new version (​https://groups.google.com/g/sage-devel/c/ij0ulNf7ZuI/m/qF0AlS9IBgAJ):

$sage -c 1+1
SageMath version 9.1, Release Date: 2020-05-20
Traceback (most recent call last):
  File "/home/conda/feedstock_root/build_artifacts/sagelib_1592119926566/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehol/bin/sage-eval", line 4, in <module>
    from sage.all import *
  File "/home/conda/feedstock_root/build_artifacts/sagelib_1592119926566/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehol/lib/python3.6/site-packages/sage/all.py", line 117, in <module>
    import sage.misc.lazy_import
  File "sage/misc/lazy_import.pyx", line 1, in init sage.misc.lazy_import (build/cythonized/sage/misc/lazy_import.c:10174)
AttributeError: type object 'sage.misc.lazy_import.LazyImport' has no attribute '__div__'

Previous update:

• #29480 - Upgrade Cython to 0.29.17 (merged in 9.2.beta1)

[dimpase]

apparently the cause for this is already found.

This works for me with no apparent issues:

​https://aur.archlinux.org/cgit/aur.git/tree/sagemath-cython-0.29.20.patch?h=sagemath-git

[mkoeppe]

0.29.21 is out. Needed by numpy branches master, maintenance/1.19.x

[mkoeppe]

Tests with 0.29.21 run at ​https://github.com/mkoeppe/sage/actions/runs/169380426

---

New commits:

​c24d109

build/pkgs/cython: Updatt to 0.29.21

[mkoeppe]

Tests look clean. Needs review

[jhpalmieri]

Works for me on OS X, seems to be needed for #27754.

[dimpase]

lgtm

[mkoeppe]

Thanks!