Make build-translation job work again

[m-aciek]

Last successful run was on 5th of April.

https://github.com/m-aciek/python-docs-pl/runs/561230778?check_suite_focus=true

It's connected with Sphinx 3.0.0 release and Makefile in Cpython's Doc directory.

[m-aciek]

BPO: Docs build error with Sphinx 3.0 due to invalid C declaration
PR: Update documentation formatting for Sphinx 3.0

[m-aciek]

Fixed with python/cpython#19442.