Broken doc build with sphinx 1.8

[anntzer]

The doc CI is broken with sphinx 1.8.

Matplotlib currently defines its own :math: role (based on mathtext rendering + png embedding), but this triggers a warning (fatal on CI for us) with sphinx 1.8 because sphinx now provides its own :math: role by default.

I removed that definition in #11928, but that doesn't actually help, because ours is slightly different: the mathtext tutorial uses the construct

.. role:: math-stix(math)
   :fontset: stix

to later be able to use math-stix meaning "render with that fontset, and this causes an error with sphinx 1.8's own math role).

I think the long-term solution is to move the mathtext tutorial to use plots to display the relevant samples, or to move matplotlib's own :math: role to something like :mplmath:, but in the mean time I can live with a warning about duplicate roles in sphinx's startup.

In the meantime we should just pin sphinx<1.8 and restore our own math role. At least fixing doc-requirements.txt would be nice for 3.0.

[tacaswell]

Fixed by #12109 / #12110