Change DataObjectHandleProperty to DataHandleProperty

Move the property declaration of data handles one level lower in the
inheritance hierarchy, i.e. from `DataObjectHandleBase` to `DataHandle`. The
only user-visible change should be the change in type of the python
configurable class from `DataObjectHandleBase` to `DataHandle`. This change
will allow other types of handles (e.g. in ATLAS) to profit from the
python type information.
12 jobs for handle_type in 28 minutes and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #8916891
cvmfs
build:gcc8:opt

00:11:42

passed #8916893
cvmfs
build:gcc9:dbg

00:18:56

passed #8916892
cvmfs
build:gcc9:opt

00:12:33

passed #8916894
cvmfs
build:gcc9:opt:python3

00:13:50

passed #8916895
check-formatting

00:00:34

 
  Test
passed #8916896
check-copyright

00:01:00

passed #8916897
cvmfs
test:gcc8:opt

00:05:24

passed #8916899
cvmfs
test:gcc9:dbg

00:09:25

passed #8916898
cvmfs
test:gcc9:opt

00:09:19

passed #8916900
cvmfs
test:gcc9:opt:python3

00:05:44

passed #8916901
cvmfs
test_public_headers_build

00:04:14

passed #8916902
test-python3

00:01:00