Description of the problem, including code/CLI snippet

The retry-after header is not always present, in which case, the client crashes.

Expected Behavior

When there isn't a retry-after header, the client uses a default retry timeout.

Actual Behavior

When there isn't a retry-after header, the client crashes

Specifications

• python-gitlab version: 1.6.0
• API version you are using (v3/v4): Whatever they use on gitlab.com
• Gitlab server version (or gitlab.com): gitlab.com