Add maintainer_can_modify parameter to create_pull

When we're creating a pull in the destination repo with credentials
that don't have write permission on the source, we always get a 422
response from GitHub. This happens because maintainer_can_modify
parameter is true by the default, and currently github3.py doesn't
support setting of maintainer_can_modify to false when creating a PR.

This commit adds this parameter to create_pull Repo method to comply
with GitHub API.

Author: Fedosin

src/github3/repos/repo.py

@@ -1151,7 +1151,9 @@ def create_project(self, name, body=None):
         return self._instance_or_null(projects.Project, json)
 
     @decorators.requires_auth
-    def create_pull(self, title, base, head, body=None):
+    def create_pull(
+        self, title, base, head, body=None, maintainer_can_modify=None
+    ):
         """Create a pull request of ``head`` onto ``base`` branch in this repo.
 
         :param str title:
@@ -1162,12 +1164,17 @@ def create_pull(self, title, base, head, body=None):
             (required), e.g., 'username:branch'
         :param str body:
             (optional), markdown formatted description
+        :param bool maintainer_can_modify:
+            (optional), Indicates whether a maintainer is allowed to modify the
+            pull request or not.
         :returns:
             the created pull request
         :rtype:
             :class:`~github3.pulls.ShortPullRequest`
         """
         data = {"title": title, "body": body, "base": base, "head": head}
+        if maintainer_can_modify is not None:
+            data["maintainer_can_modify"] = maintainer_can_modify
         return self._create_pull(data)
 
     @decorators.requires_auth

tests/unit/test_repos_repo.py

@@ -341,6 +341,19 @@ def test_create_pull(self):
             self.instance.create_pull(**data)
             pull.assert_called_once_with(data)
 
+    def test_create_pull_maintainer_can_modify(self):
+        """Verify maintainer_can_modify option for creating a pull request."""
+        data = {
+            "title": "foo",
+            "base": "master",
+            "head": "feature_branch",
+            "body": "body",
+            "maintainer_can_modify": False
+        }
+        with unittest.mock.patch.object(Repository, "_create_pull") as pull:
+            self.instance.create_pull(**data)
+            pull.assert_called_once_with(data)
+
     def test_create_pull_from_issue(self):
         """Verify the request for creating a pull request from an issue."""
         with unittest.mock.patch.object(Repository, "_create_pull") as pull: