Comment 4 for bug 1584858

the ovs requirement in question was removed on ryu master. [1]
and a patch release including the change is now available on pypi. [2]
so you can now add !=4.2.1 to requirements and use 4.2.2.

[1] https://github.com/osrg/ryu/commit/9e3aaaf2d812a08201ff615bec0317d80840f094
[2] https://pypi.python.org/pypi/ryu/4.2.2