MathJax fonts not always synced

[lochsh]

For some reason, the MathJax fonts in strictdoc/export/html/_static_extra/mathjax/output/chtml/fonts/woff-v2/ are not always synced with the output directory/_static/.... (When --enable-mathjax is set, of course.)

I think it might be a cache invalidation issue stemming from using https://docs.python.org/3/distutils/apiref.html#distutils.dir_util.copy_tree with update=True.

I'll see if I can get some reliable reproductions of this and will update the issue.