Merge pull request #9 from bbc/philipn-pin-yarl

Pin yarl before 1.6.0 to keep query string escaping working

Author: Philip de Nier

constraints.txt

@@ -23,3 +23,6 @@ s3transfer>=0.3.0,<0.4.0
 # Requirements due to sanic
 httpx==0.9.3
 sanic==19.12.2
+
+# yarl 1.6.0 does not unescape query strings like 1.5.1 does
+yarl<1.6.0