[ROOT-9880] Prevent crash in Py3.7 when adding TVector3 pythonization
[ROOT-9880] Prevent crash in Py3.7 when adding TVector3 pythonization

The Python error indicator needs to be cleared before trying to
retrieve __name__, if there was a previous (unsuccessful) retrieval
of __cppname__.

root-mirror/root,olifre/root,karies/root,root-mirror/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,olifre/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,olifre/root,olifre/root