Comment 5 for bug 1883659

I am able to make the problem go away with this extra setting in neutron.conf:

[keystone_authtoken]
memcache_use_advanced_pool = True

This is the documentation for this setting:

# (Optional) Use the advanced (eventlet safe) memcached client pool. The
# advanced pool will only work under python 2.x. (boolean value)

This description dates from 2016. For now I haven't seen any issue enabling this setting with Python 3.