Fixes #1197

Switch to objgraph to handle the measurement for us. That cleared up a
few of the obscure issues with references to
functions/getset_descriptors and the like. (Possibly because it keys
by string names and we were keeping type objects alive.)

Many of the real failures were due to re-using exception instances,
which is bad because of chaining.

Most of the @ignore_leakcheck are for performance, only one is for a
real issue---and that test was skipped already on CI anyway for being
too flaky.