MRG, DOC: Bump theme version

[drammock]

a few recent changes to pydata-sphinx-theme allow us to simplify things on our end:

• they've implemented a comprehensive system-native font stack. Consequently I have un-vendored our webfonts to speed up page load times. Our site now continues to use Source Sans Pro and Source Code Pro if they're present on the user's system, and will fall back to a system-appropriate default otherwise.
• they've improved how navbar icon links (top right corner) are handled. This greatly simplifies our template override doc/_templates/docs-navbar.html

Other changes:

• CSS variables defined by the theme now all start with --pst-
• adds icon link to our Discord server
• small CSS fix so our in-text sidebar notes get done properly
• update the font versions that Circle uses

[hoechenberger]

The "sidebar fix" from ff23d41 could use a little margin at the bottom:

The padding is also different between top and bottom.

Also wondering if offering to make this box collapsible could be a good idea?

[hoechenberger]

How about:

.sidebar {
  background-color: rgba(var(--pst-color-info)), 0.1);
  border: 1px solid rgb(var(--pst-color-info)));
  border-radius: 4px;
  padding: 1em 1em 0 1em;
  margin-bottom: 2rem;
  width: 90%;
}

?

[drammock]

hmm, circle build is not matching what I saw locally @hoechenberger. I'm not surprised you had complaints about it :) I'll see what went wrong.

[drammock]

OK well, I am stumped. the .. sidebar:: directive on the sphinx site itself is rendered as a div tag with class sidebar. The same happens locally for me. But on our Circle server, it gets rendered as an aside tag. A solid hour of comparing package versions, searching docs, and grepping source trees has me no closer to figuring out why this difference exists or what is causing it. So I'm just going to apply the CSS style to both div.sidebar and aside.sidebar and be done with it.

[drammock]

OK, it's working now @hoechenberger:

[drammock]

turns out that div -> aside was a change in the HTML5 writer of docutils v0.17.

[drammock]

CI failures are LGTM's "cannot build merge commit" snafu, and a failure of test_make_screenshot[pyvista] on macOS py3.8

Ready for review/merge

[agramfort]

thx @drammock