branching: merge stable into default (preserving test changes on default)

CORE-TEST-OUTPUT-UPDATE: 768056549737
CORE-TEST-OUTPUT-UPDATE: dc00324e80f4
8 jobs for topic/default/release-prep in 8 minutes and 51 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #175208
checks-py2

00:04:27

passed #175209
checks-py3

00:04:00

passed #175214
doc

00:02:19

manual #175207
allowed to fail manual
pytype
passed #175210
tests-py2-cext

00:08:51

passed #175211
tests-py2-pure

00:05:48

passed #175212
tests-py3-cext

00:08:50

passed #175213
tests-py3-pure

00:06:15