Require the redis module for test test_timeouts_in_url_coerced (#4847)

The test requires the redis module to be installed. It is referenced in
the function RedisBackend._params_from_url().

Fixes AppVeyor build failures.

t/unit/backends/test_redis.py

@@ -234,6 +234,7 @@ class test_RedisBackend:
         assert x.connparams['socket_timeout'] == 30.0
         assert x.connparams['socket_connect_timeout'] == 100.0
 
+    @skip.unless_module('redis')
     def test_timeouts_in_url_coerced(self):
         x = self.Backend(
             ('redis://:bosco@vandelay.com:123//1?'