PyUtils: Go back to using regex for reading the number of entries in AthFileLite
This reverts !69921 (merged) and goes back to what I originally implemented in !67863 (merged) but uses the APRDefaults
in the regex for the collection names. It looks like the former broke a unit-test where we were reading an ESD
from 2011, which has POOLContainer_DataHeader
instead of POOLContainer
for the DataHeader
.
Closes ATEAM-968
cc: @gemmeren