This adds a Make target that should simplify the common task of
upgrading a single dependency. Sometimes people manually edit the pin
files, which we would like to avoid; hopefully this will make it
easier for them to do the right thing.
The GitHub workflow should also make it easier for people on Mac to
recompile requirements in a Linux environment, reducing the number of
times spurious dependency changes show up in the pin files (due to
OS-dependent requirements.)
Also, separate upgrade/downgrade instructions and simplify the latter.
(Min constraints are rare and we usually move beyond them quickly.)
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