Fix sorting released PEP-440 style versions
Fixes:
[cburr@lxplus724]~% lb-dirac --list
Traceback (most recent call last):
File "/cvmfs/lhcb.cern.ch/lib/var/lib/LbEnv/2075/unstable/linux-64/bin/lb-dirac", line 10, in <module>
sys.exit(lb_dirac())
File "/cvmfs/lhcb.cern.ch/lib/var/lib/LbEnv/2075/unstable/linux-64/lib/python3.8/site-packages/LbDiracWrappers/__init__.py", line 177, in lb_dirac
print(*sort_versions(LHCBDIRAC_VERSIONS), sep="\n")
File "/cvmfs/lhcb.cern.ch/lib/var/lib/LbEnv/2075/unstable/linux-64/lib/python3.8/site-packages/LbDiracWrappers/__init__.py", line 88, in sort_versions
return sorted(parsedVersions, key=parsedVersions.get, reverse=True)
TypeError: '<' not supported between instances of 'int' and 'NoneType'