These are not the latest versions, but pip-tools 3.9.0 or 4.0.0 fails
to resolve dependencies from Git URLs:
pip._internal.exceptions.DistributionNotFound: No matching distribution found for zulip==0.6.1_git (from -r requirements/common.in (line 135))
while pip 19.2 breaks pip-tools 3.8.0:
TypeError: __init__() got an unexpected keyword argument 'find_links'
Fixes#10802.
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>