When building the SR plantuml with only shacache is accessible. plantuml wants to install Pango, and in the Pango Makefiles, it needs to invoke glib-mkenums. Which the glib-mkenums needs the python2.7. So here to provide Python 2.7 by default. See the full error log here:
https://erp5js.nexedi.net/#/test_result_module/20220811-21A210A8D/2
Regarding the testfixtures, the version in the PyPI index file is 6.11.0. Using version 6.11` will cause it to fail to find the cached package on the shacache. This is because PyPI has an "autocorrection" functionality and our dedicated code which is used to cache the PyPI package on shacache doesn't have this ability. The simples solution is to use the exact version number same as the PyPI index.