Add support for Workflows (#1496)

To start supporting GitHub Actions, add the first part of that,
Workflow, which encapsulates a workflow over the API, along with
two methods on Repository to query them.

github/Workflow.pyi

@@ -0,0 +1,27 @@
+from datetime import datetime
+from typing import Any, Dict
+
+from github.GithubObject import CompletableGithubObject
+
+class Deployment(CompletableGithubObject):
+    def __repr__(self) -> str: ...
+    def _initAttributes(self) -> None: ...
+    def _useAttributes(self, attributes: Dict[str, Any]) -> None: ...
+    @property
+    def id(self) -> int: ...
+    @property
+    def name(self) -> str: ...
+    @property
+    def path(self) -> str: ...
+    @property
+    def state(self) -> str: ...
+    @property
+    def created_at(self) -> datetime: ...
+    @property
+    def updated_at(self) -> datetime: ...
+    @property
+    def url(self) -> str: ...
+    @property
+    def html_url(self) -> str: ...
+    @property
+    def badge_url(self) -> str: ...