Bug#1012386: python-seqcluster: FTBFS: ModuleNotFoundError: No module named 'attr'

[Bastian Germann]

Control: notfound -1 python-seqcluster/1.2.8+ds-2
Control: close -1

This was obviously an issue in myst_parser module which is not part of 
python-seqcluster.
It is already fixed, so the FTBFS is as well.