Sphinx: Could not import extension sphinx.ext.pngmath (exception: No module named 'sphinx.ext.pngmath')

[bbg221]

exception occurred when I use 'make html' to build a doc which I download from opencv git-project.
Could not import extension sphinx.ext.pngmath (exception: No module named 'sphinx.ext.pngmath')

I use 'pip install -U sphinx' to install it and its version is v2.2.0 and my python version is 3.7.0 , OS is win7

when I use sphinx to make html, it faild.
I write 'import sphinx' and 'import sphinx.ext.doctest' into python command line. all these two lines are worked.
but I write 'import sphinx.ext.pngmath', it faild. show the error: ModuleNotFoundError: No module name 'sphinx.ext.pngmath'

[tk0miya]

pngmath extension was deprecated at Sphinx-1.6 and removed at 1.8. Now sphinx.ext.imgmath is a successor of pngmath. Please update your configuration list.
Thanks,