The function `register` of the `webbrowser` module was updated a little while back to use the newish keyword-only argument syntax; however, the function `_synthesize` (also in `webbrowser`) is still using the outdated positional arguments only calling convention; leading a pair of tests in `Lib/tests/test_webbrowser.py` to fail.  I've issued a PR that fixes the function call to use the more modern calling convention.