Pin webargs version <6.0.0

Webargs >= 6.0.0 introduced a breaking change where schema fields may
not specify a location any longer. We need to change our code to specify
per-location schemas (easy) but aiohttp-apispec is not ready yet until
maximdanilchenko/aiohttp-apispec/83 is fixed.

This change pins the webargs version to something below 6.0.0 so that
things are not borken.

Fixes ai4os#82
Sem-Ver: bugfix

Author: alvarolopez

requirements.txt

@@ -14,6 +14,6 @@ aiohttp-apispec>=2.2.0
 
 werkzeug>=0.16.0
 marshmallow>=3.2.1
-webargs>=5.5.2
+webargs>=5.5.2,<6.0.0
 
 jsonschema>=2.6.0