www/py-flask-dance fails docs build after distutils deprecation

Hi,

For the last few months I've been receiving pkg-fallout emails because 
when distutils was deprecated one of the ports used for building docs on 
a port I own requires it as RUN_D. The port 
is*/textproc/py-sphinxcontrib-seqdiag/*. The bug has been sitting since 
march, but I have been getting daily pkg-fallout emails which is quite 
annoying. I created a patch, can it be committed?

285339 – textproc/py-sphinxcontrib-seqdiag: missing run depend 
py-setuptools breaking www/py-flask-dance 
<https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=285339>

Thanks