Comment # 14 on bug 1166965 from Michal Kube��ek

I checked /var/log/zypp/history in the Tumbleweed chroot I'm using for
config updates, I see

  2019-12-09 08:47:07|install|libpython3_7m1_0|3.7.3-1.4
  ...
  2020-03-09 06:36:00|install|vim|8.2.0348-1.1
  ...
  2020-03-13 08:02:04|install|libpython3_8-1_0|3.8.2-1.1

So it seems python 3.8 wasn't available in TW repositories until after 
March 9. On the other hand, there was an update of python3-Sphinx before
that:

  2020-01-03 08:31:14|install|python3-Sphinx|2.3.1-1.1
  2020-03-09 06:34:55|install|python3-Sphinx|2.4.3-1.1

I also ran "zypper dup" on 2020-03-02 but that that was quite early so it's
possible that the first failing builds already used python3-Sphinx 2.4.3.

So it indeed seems that the problem started between sphinx 2.3.1 and 2.4.3.