Error messages are not converted from bytes to str in Python 3 #616

Description of the problem, including code/CLI snippet

When trying to create webhooks on a repository with invalid URLs (like 127.x.x.x), GitLab returns HTTP 422 with a JSON body : {"error":"Invalid url given"}. Under Python 3, the GitlabCreateError does not convert from bytes to str :

from gitlab import Gitlab
from gitlab.exceptions import GitlabCreateError

gl = Gitlab('https://gitlab.com', oauth_token="<TOKEN>")
project = gl.projects.get('<PROJECT>')

try:
    project.webhooks.create({'url': 'http://127.0.0.1/'})
except GitlabCreateError as e:
    print(str(e))

Expected Behavior

Under both Python 2 and 3, the above snippet should print

422: {"error": "Invalid url given"}

Actual Behavior

Under Python 2, it works as expected. Under Python 3 only, this prints

422: b'{"error": "Invalid url given"}'

Specifications

• python-gitlab version: 1.5.1
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): GitLab.com (11.4.0-rc7-ee)