Fix #382 - boto3 compatibility

* Add support for PyYaml5.1
* Unpin requests in Tox
* Unpin urllib3 in Tox
* Unpin Flask in Tox
* Add env vars to Tox for boto3 tests

vcr/stubs/__init__.py

@@ -170,6 +170,7 @@ class VCRConnection(object):
             self._port_postfix(),
             url,
         )
+        log.debug("Absolute URI: %s", uri)
         return uri
 
     def _url(self, uri):