Nixpkgs: python38Packages.bleach build broken

[ajs124]

Describe the bug
2012dd5734ef0f15a68794020283b112625b3f95 broke the build of python38Packages.bleach, see here, python37 is still fine.
This is probably an upstream issue in either bleach or python.
bleach master still has this behavior, though.
They do test on 3.8, that is however 3.8.0, not 3.8.1 as of writing this.

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: python38Packages.bleach
# a list of nixos modules affected by the problem
module:

cc @FRidh

[pacien]

Upstream bugs:

• https://github.com/mozilla/bleach/issues/503
• https://bugs.python.org/issue27657