Install cython+ via its sources

.gitmodules

+[submodule "cython"]
+	path = cython
+	url = https://lab.nexedi.com/nexedi/cython.git

cython @ 03d7c50c

+Subproject commit 03d7c50ce1617f8cb49af8eab38fd8a7b640a633

requirements.txt

-git+https://lab.nexedi.com/nexedi/cython.git@03d7c50ce1617f8cb49af8eab38fd8a7b640a633#egg=Cython
 ipython==7.22.0
 jupyter==1.0.0
 line-profiler==3.1.0
 matplotlib==3.4.1
 numpy==1.20.2
+-e git+https://lab.nexedi.com/nexedi/cython.git@03d7c50ce1617f8cb49af8eab38fd8a7b640a633#egg=Cython&subdirectory=../../cython