Bump ryu to 4.18 to avoid pip import issues

Importing pip in code is apparently not supported, but the ryu
library is using it internally. Version 4.18 included a change
to only import pip where needed in a specific method that we
appear to not call from any OpenStack code. Bumping u-c to pick
up this change and test with affected projects.

Pip issue with more details:
pypa/pip#5081

Change-Id: Ia358176cd9d590ac153fcf6bd9da34e2425ff3ee
Depends-on: https://review.openstack.org/554338

upper-constraints.txt

@@ -368,7 +368,7 @@ rfc3986===0.4.1
 rjsmin===1.0.12
 rsa===3.4.2
 rtslib-fb===2.1.62
-ryu===4.10
+ryu===4.18
 scandir===1.4
 scikit-learn===0.18.1
 scipy===0.18.1