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 #82 (closed)