BREAKING CHANGE: GraphQL.execute() no longer accepts graphql.Source

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

black changed how some code was formatted in v26

Passing `start_branch` as kwargs results in it being passed as query
argument to the API:

```
send: b'PUT /api/v4/projects/12345678/repository/files/readme.txt?start_branch=main

send: b'{"file_path": "readme.txt", "branch": "new_branch", "content":
"Modified contents", "commit_message": "File was modified for this new
branch"}'
```

which results in error being returned:

```
{"message":"You can only create or edit files when you are on a branch"}
```

It should instead be sent a body attribute, which succeeds in creating
the branch during the save.

To be sent as body attribute it must be specified as concrete function
argument and class attribute instead of just using kwargs

Closes: #3318

While `_repr_attr=` is given to specify the attribute to use to repr the
object, that object attribute itself is not actually defined, so tools
like PyRight/MyPy complain that:

```
Type of "path_with_namespace" is unknown
```

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Prior commit 39975d5 updated black to
v26 but did not update `.pre-commit-config.yaml`. This resolves that.