Allow self as an argument to url_for

[pgjones]

This makes the Flask.url_for self argument positional only (Flask supports Python 3.8+) thereby restoring the ability to pass self as a value argument to url_for.

Closes #5258

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.