GeezerVison

WITH_CLEANUP only silenced the exception when __exit__ returned the True singleton, so a context manager whose __exit__ returns e.g. 1 let the exception through.

CPython 3.4 tests the result with PyObject_IsTrue and jumps to the error path if that raises. This uses py.ObjectIsTrue, which follows PyObject_IsTrue (including __bool__/__len__ on class instances since #222), and returns its error.

Tests: vm/tests/with.py now checks that true non-bool results (1, "potato", [0], 0.5, object()) silence the error, false ones (None, False, 0, "", [], 0.0, and instances with a false __bool__ or zero __len__) don't, and that an exception raised by the result's __bool__ propagates. The new cases fail on main and pass here; the file also passes under CPython. go test -race ./... passes on Go 1.18, 1.19 and 1.27.

Fixes #267.