tox.ini: merge py3-{core,server,db} test environments
DRY is better (stepping stone to swh-environment#2107 (closed))
Test Plan
tox -> no changes
Migrated from D2327 (view on Phabricator)
DRY is better (stepping stone to swh-environment#2107 (closed))
tox -> no changes
Migrated from D2327 (view on Phabricator)