Github actions: black version mismatch

[bwohlberg]

I just had a PR fail the lint test, apparently due to a mismatch between black versions being used locally (19.10b0) and on github (presumably 21.9b0, the latest stable release). The local version of black remained at 19.10b0 after a conda update, despite 21.9b0 begin available via conda, presumably due to some dependency conflicts. It would be useful to try to figure out a strategy for avoiding such a situation in the future.

[Michael-T-McCann]

There are two issue in one here:

1. Your version of black is older than dev_requirements.txt specifies. Unfortunately, when we update the version of a dependency, everyone needs to update their environment to meet the version requirement. I don't see a way around that, we'll just try not to do it too often.

2. Because the black dependency was black >= 21.6b0, the CI would end up with the latest version of black. That's not desirable because, e.g., version 22.1 (when/if it appears) and version 21.6 would both meet the requirement but cause lint errors because they (may) format differently. To guard against this, I changed the requirement to black >= 21.6b0, == 21.* in 93c8a8c.