PullRequest.delete_branch: fix the remaining pull requests check (PyGithub#3063)

### Problem
`delete_branch(force=False)` raises a `RuntimeError` if the repository
has any open pull requests because the current check used for remaining
open pull requests returns all open pull requests for the repository
instead of only the open pull requests associated with the pull
request's branch.

### Fix
Updating this to use the format of `user:ref-name` or
`organization:ref-name` appropriately gets open pull requests for the
pull request's branch.

```
remaining_pulls = self.head.repo.get_pulls(head=f"{self.head.repo.owner.login}:{self.head.ref}")
```

> [!NOTE]
> See: PyGithub#3031
> From docs:
https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#list-pull-requests
![Screenshot 2024-10-17 at 3 13
20 PM](https://github.com/user-attachments/assets/d0fbe8e9-09f3-42cc-8dd9-716418de2387)

Author: fetsko

github/PullRequest.py

@@ -777,7 +777,7 @@ def delete_branch(self, force: bool = False) -> None:
         Convenience function that calls :meth:`GitRef.delete` :rtype: bool.
         """
         if not force:
-            remaining_pulls = self.head.repo.get_pulls(head=self.head.ref)
+            remaining_pulls = self.head.repo.get_pulls(head=f"{self.head.repo.owner.login}:{self.head.ref}")
             if remaining_pulls.totalCount > 0:
                 raise RuntimeError(
                     "This branch is referenced by open pull requests, set force=True to delete this branch."

tests/ReplayData/PullRequest.testDeleteBranch.txt

@@ -13,7 +13,7 @@ https
 GET
 api.github.com
 None
-/repos/austinsasko/PyGithub/pulls?head=revert-20-revert-19-revert-18-revert-16-revert-15-revert-14-revert-13-revert-12-revert-11-revert-10-revert-9-revert-8-revert-7-revert-6-revert-5-revert-4-revert-3-revert-2-revert-1-testing&per_page=1
+/repos/austinsasko/PyGithub/pulls?head=austinsasko%3Arevert-20-revert-19-revert-18-revert-16-revert-15-revert-14-revert-13-revert-12-revert-11-revert-10-revert-9-revert-8-revert-7-revert-6-revert-5-revert-4-revert-3-revert-2-revert-1-testing&per_page=1
 {'Authorization': 'Basic login_and_password_removed', 'User-Agent': 'PyGithub/Python'}
 None
 200

tests/ReplayData/PullRequest.testDeleteOnMerge.txt

@@ -35,7 +35,7 @@ https
 GET
 api.github.com
 None
-/repos/austinsasko/PyGithub/pulls?head=revert-20-revert-19-revert-18-revert-16-revert-15-revert-14-revert-13-revert-12-revert-11-revert-10-revert-9-revert-8-revert-7-revert-6-revert-5-revert-4-revert-3-revert-2-revert-1-testing&per_page=1
+/repos/austinsasko/PyGithub/pulls?head=austinsasko%3Arevert-20-revert-19-revert-18-revert-16-revert-15-revert-14-revert-13-revert-12-revert-11-revert-10-revert-9-revert-8-revert-7-revert-6-revert-5-revert-4-revert-3-revert-2-revert-1-testing&per_page=1
 {'Authorization': 'Basic login_and_password_removed', 'User-Agent': 'PyGithub/Python'}
 None
 200