python3Packages.cytoolz: fix build

cytoolz has a test failure with Python 3.7 [0]. There is a debian patch
for it, which didn't make it into upstream yet [1].

[0] https://github.com/pytoolz/cytoolz/issues/122
[1] https://salsa.debian.org/python-team/modules/python-cytoolz/raw/5ce4158deefc47475d1e76813f900e6c72ddcc6e/debian/patches/py37.patch