Use pyupgrade to convert simple string formatting to use f-string
syntax. pyupgrade is intentionally timid and will not create an f-string
if it would make the expression longer or if the substitution parameters
are anything but simple names or dotted names.
This essentially allows me to do an overall check general check by running the tests using pytest's `-k basic` syntax. Given that I like running tests often and that, in general, I make typos more often than changes that break core functionality, I think this will reduce cycle times for me.