Ternary operators in GitHub Actions syntax are pretty ugly and hard to
read: `inputs.working-directory == '' && '.' ||
inputs.working-directory` means "if the condition is true, use `'.'` and
otherwise use the expression after the `||`".
This PR performs the ternary as few times as possible, assigning its
outcome to an env var we can then reuse as needed.
Only lint on the min and max supported Python versions.
It's extremely unlikely that there's a lint issue on any version in
between that doesn't show up on the min or max versions.
GitHub rate-limits how many jobs can be running at any one time.
Starting new jobs is also relatively slow, so linting on fewer versions
makes CI faster.
Predrag Gruevski