Address GitHub deprecation not yet fixed in social-auth-core v3.2.0

Reported in:
python-social-auth/social-core#430

Patch in:
python-social-auth/social-core#428

tcms_github_app/backend.py

@@ -1,3 +1,5 @@
+from six.moves.urllib.parse import urljoin
+
 from social_core.backends.github import GithubOAuth2
 from social_core.utils import handle_http_errors
 
@@ -8,6 +10,12 @@ class GithubAppAuth(GithubOAuth2):  # pylint: disable=abstract-method
     """GitHub App OAuth authentication backend"""
     name = 'github-app'
 
+    # copied from https://github.com/python-social-auth/social-core/pull/428
+    # and must be removed once we've got a newer social-auth-core
+    def _user_data(self, access_token, path=None):
+        url = urljoin(self.api_url(), 'user{0}'.format(path or ''))
+        return self.get_json(url, headers={'Authorization': 'token {0}'.format(access_token)})
+
     def validate_state(self):
         """
             Scenario 1: user clicks an icon/button on your website and