XRootD dependency

Since recently, Allen standalone requires the python bindings of XRootD, due to using PyConf:

https://gitlab.cern.ch/lhcb/LHCb/-/blob/master/PyConf/python/PyConf/utils.py#L11

At least locally from a macos machine, the python bindings of XRootD don't seem to be installed automatically when installing it through brew. I did find the following on this topic:

https://frankenthal.dev/post/pyxrootd-bindings-in-macos/

The procedure and requirements of Allen standalone should be updated.