sphinx.ext.pngmath problem on readthedocs

[aewallin]

the documentation will not build at the moment...

Running Sphinx v1.8.5
loading translations [en]... done

Traceback (most recent call last):
File "/home/docs/checkouts/readthedocs.org/user_builds/allantools/envs/latest/lib/python2.7/site-packages/sphinx/cmd/build.py", line 303, in build_main
args.tags, args.verbosity, args.jobs, args.keep_going)
File "/home/docs/checkouts/readthedocs.org/user_builds/allantools/envs/latest/lib/python2.7/site-packages/sphinx/application.py", line 228, in init
self.setup_extension(extension)
File "/home/docs/checkouts/readthedocs.org/user_builds/allantools/envs/latest/lib/python2.7/site-packages/sphinx/application.py", line 449, in setup_extension
self.registry.load_extension(self, extname)
File "/home/docs/checkouts/readthedocs.org/user_builds/allantools/envs/latest/lib/python2.7/site-packages/sphinx/registry.py", line 475, in load_extension
raise ExtensionError(__('Could not import extension %s') % extname, err)
ExtensionError: Could not import extension sphinx.ext.pngmath (exception: No module named pngmath)

Extension error:
Could not import extension sphinx.ext.pngmath (exception: No module named pngmath)

[aewallin]

fix is:
sphinx.ext.imgmath