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 inCHANGES.rst summarizing the change and linking to the issue.
• Add.. versionchanged:: entries in any relevant code docs.
• Runpre-commit hooks and fix any issues.
• Runpytest andtox, no tests failed.