Release Docrepr 0.2.1

[CAM-Gerlach]

Since 0.2.0 was the first Docrepr release in quite a while, and there are some moderately to perhaps more critical bugs remaining for 0.2.1, we'll probably want to release 0.2.1 as soon as a couple weeks after 0.2.0 is out on PyPI and conda, particularly if any important/critical bugs are found. I opened this issue as a general venue to discuss and track progress toward the next patch release. The plan after this is to release 0.3.0 dropping Python 3.6 support and possibly larger changes, as well as any further bugfix releases needed on the 0.2.x branch

Hard-blockers:

• Any important or critical bugs discovered in the weeks after the 0.2.0 release
• docrepr v0.2.0 conda-forge/docrepr-feedstock#3 merged and out for at least a week or two

Soft blockers:

• Plots don't render at all on Windows  #38
• Avoid require pytest-asyncio CLI args/config if not running visual regression tests #45
• Significant visual regression in collapse headings mode with Sphinx 4 #46

Targeted for v0.2.0, but can be deferred to a later patch or minor release:

• Add Pre-Commit suite and CIs and upgrade code accordingly #47
• Update old vendored CSS and JS to more modern, supported and secure versions #48

Other misc items:

• Update release guide with minor tweaks (messages, etc) from 0.2.0 experiance
• Make sure to bump HTML version