Description of the problem, including code/CLI snippet

Commit object does not seems to provide information about the signature of the commit.

Expected Behavior

Be able to get the signature status of a commit, as per https://docs.gitlab.com/ee/api/commits.html#get-gpg-signature-of-a-commit

Actual Behavior

Not able to get the signature status of a commit

Specifications

• python-gitlab version: 2.1.2
• API version you are using (v3/v4): v4
• Gitlab server version (or gitlab.com): 12.7.6