Cannot compile documentation (sphinx.ext.pngmath is deprecated)

[dragokas]

When attempt to run:

docs/make.bat html

exception: No module named 'sphinx.ext.pngmath'

See: sphinx-doc/sphinx#6182

[StoicLoofah]

@dragokas can you confirm whether the branch below fixes your problem?

[dragokas]

Build is success using @cclauss branch.