Restore multiproc tests in Python 3.8+

[effigies]

#3154 saw some reductions in testing scope to avoid hanging tests, which presents us with Python 3.7's EOL as a kind of bit-rot cliff.

It looks like we're not the only ones to have had this problem (scipy/scipy#11835 / scipy/scipy#11836), so we should make another attempt to recover these tests.

HT @tclose for finding these issues.