Add: delete_all on labels

Unlike the description says within set_all labels are not overriden but
labels that are not within the POST data but are on the PR stay.

For the use case that an action wants to override managed labels it
needs the possibility to delete previous labels.

For that the delete_all method is introduced.

pontos/github/api/labels.py

@@ -45,11 +45,31 @@ async def get_all(
             for label in data:
                 yield label["name"]  # type: ignore
 
+    async def delete_all(self, repo: str, issue: Union[int, str]) -> None:
+        """
+        Deletes all labels in the issue/pr.
+
+        Args:
+            repo:   GitHub repository (owner/name) to use
+            issue:  Issue/Pull request number
+
+        Example:
+            .. code-block:: python
+
+                from pontos.github.api import GitHubAsyncRESTApi
+
+                async with GitHubAsyncRESTApi(token) as api:
+                    await api.labels.delete_all("foo/bar", 123)
+        """
+        api = f"/repos/{repo}/issues/{issue}/labels"
+        response = await self._client.delete(api)
+        response.raise_for_status()
+
     async def set_all(
         self, repo: str, issue: Union[int, str], labels: Iterable[str]
     ) -> None:
         """
-        Set labels in the issue/pr. Existing labels will be overwritten
+        Set labels in the issue/pr.
 
         Args:
             repo:   GitHub repository (owner/name) to use

tests/github/api/test_labels.py

@@ -46,6 +46,16 @@ async def test_get_all(self):
             params={"per_page": "100"},
         )
 
+    async def test_delete_all(self):
+        response = create_response()
+        self.client.delete.return_value = response
+
+        await self.api.delete_all("foo/bar", 123)
+
+        self.client.delete.assert_awaited_once_with(
+            "/repos/foo/bar/issues/123/labels"
+        )
+
     async def test_set_all(self):
         response = create_response()
         self.client.post.return_value = response