Update changelog and next milestone

Signed-off-by: Ulysses Souza <ulysses.souza@docker.com>

docker/version.py

-version = "3.8.0-dev"
+version = "4.0.0-dev"
 version_info = tuple([int(d) for d in version.split("-")[0].split(".")])

docs/change-log.md

 Change log
 ==========
 
+3.7.1
+-----
+
+[List of PRs / issues for this release](https://github.com/docker/docker-py/milestone/58?closed=1)
+
+### Bugfixes
+
+* Set a different default number (which is now 9) for SSH pools
+* Adds a BaseHTTPAdapter with a close method to ensure that the
+pools is clean on close()
+* Makes SSHHTTPAdapter reopen a closed connection when needed
+like the others
+
 3.7.0
 -----