[pkg] Add missing entries to MANIFEST.in
Detected by running `check-manifest
<https://github.com/mgedmin/check-manifest>`_. Add a tox environment
accordingly.
Closes #15229018.
change_attribute_type('CWCache', 'timestamp', 'TZDatetime')
change_attribute_type('CWUser', 'last_login_time', 'TZDatetime')
change_attribute_type('CWSource', 'latest_retrieval', 'TZDatetime')
drop_attribute('CWSource', 'synchronizing')
add_attribute('CWSource', 'in_synchronization')