caldav carddav test pipelines require python v2

[phil-davis]

The test pipelines for caldav and carddav require python v2. That is because the upstream repo still does not support python v3.

This upstream PR attempts to support python v3: apple/ccs-caldavtester#40

The scripts that run these tests are in https://github.com/owncloud/core/tree/master/apps/dav/tests/ci

See the dav() function in https://github.com/owncloud/core/blob/master/.drone.star for details of how it runs in CI.

We had to explicitly install python v2 in the owncloudci/php docker image because of this. See owncloud-ci/php#139

Sort out how we can get these tests running with python v3.