STYLE: Specify target-version for black
The `target-version` option explicitly tells `black` that it can use Python features from the supplied versions.  The default is per-file detection by looking for the presence specific syntax, e.g. f-strings would indicate py36.

Currently `black` does not detect this for us since we just bumped to py36, so detection will not occur until f-strings are added, which would in turn necessitate these changes as part of the f-string PRs, making them a bit more convoluted than needed.  This resolves that issue by explicitly telling `black` our supported versions and enforcing those changes for the entire codebase in this PR.