RTD PR builds are timing out

[dcherian]

See https://readthedocs.org/projects/xray/builds/

There's no useful information in the logs AFAICT: e.g. https://readthedocs.org/projects/xray/builds/11504571/

[keewis]

This might be because of a matplotlib warning, see my own RTD setup. Not sure why the main setup times out while mine fails cleanly.

Edit: we don't even use epoch2num, that's called by pandas. I think we can just use :okwarning: until there's a new release of matplotlib and/or pandas.