fix(pulls): handle repo["owner"] == None

`owner` is null if the user who opened the PR
no longer exists on GitHub.

Author: caugner

src/github3/pulls.py

@@ -71,7 +71,9 @@ def _update_attributes(self, dest):
         repo = dest.get("repo")
         if repo:
             self._repo_name = repo.get("name")
-            self._repo_owner = repo["owner"]["login"]
+            self._repo_owner = repo.get("owner")
+            if self._repo_owner:
+                self._repo_owner = self._repo_owner["login"]
             self.repository = ShortRepository(repo, self)
         self.repo = (self._repo_owner, self._repo_name)