Pin redis for fakeredis

It looks like `poetry install` will install the latest version of redis
(v5.0.4) instead of what pip installs (v2.10.6). Unfortunately this
means that the old version of fakeredis that is pinned in the
requirements.txt will not work properly.

Fixes #903

requirements.txt

@@ -1,7 +1,7 @@
 six
 warcio>=1.7.1
 requests
-redis
+redis==2.10.6
 jinja2>=3.1.2
 surt>=0.3.1
 brotlipy