When pytest is not installed, actually fail with the intended error.
When pytest is not installed, actually fail with the intended error.

Interestingly, man sh claims:

              -u      Treat unset variables as an error when performing param-
                      eter expansion.  If expansion is attempted on  an  unset
                      variable, the shell prints an error message, and, if not
                      interactive, exits with a non-zero status.

But in practice there was no error message, so this manifested as ./check.sh simply doing nothing, apparently (to a human observer) exiting correctly without output.

Also use the python provided to do the check for pytest, and give a more helpful message.

probcomp/bayeslite,probcomp/bayeslite