Fix GaudiPython followin IEventProcessor interface change

The interface change in !877 (merged) causes errors in some of LHCb's tests that rely on GaudiPython. We should fix that.

An example is the Tesla.containers test in DaVinci, e.g. https://lhcb-nightlies.cern.ch/logs/tests/nightly/lhcb-run2-patches/422/x86_64-slc6-gcc8-opt/DaVinci/#failures_132. More specifically, the stderr reads

Traceback (most recent call last):
  File "/workspace/build/DaVinci/Phys/Tesla/tests/options/containers.py", line 104, in <module>
    gaudi.executeEvent()
  File "/workspace/build/Gaudi/InstallArea/x86_64-slc6-gcc8-opt/python/GaudiPython/Bindings.py", line 1318, in executeEvent
    return self._evtpro.executeEvent()
TypeError: StatusCode IEventProcessor::executeEvent(EventContext&& ctx) =>
    takes at least 1 arguments (0 given)