Bump 3.1.1

Signed-off-by: Joffrey F <joffrey@docker.com>

docker/version.py

-version = "3.2.0-dev"
+version = "3.1.1"
 version_info = tuple([int(d) for d in version.split("-")[0].split(".")])

docs/change-log.md

 Change log
 ==========
 
+3.1.1
+-----
+
+[List of PRs / issues for this release](https://github.com/docker/docker-py/milestone/46?closed=1)
+
+### Bugfixes
+
+* Fixed a bug that caused costly DNS lookups on Mac OSX when connecting to the
+  engine through UNIX socket
+* Fixed a bug that caused `.dockerignore` comments to be read as exclusion
+  patterns
+
 3.1.0
 -----