Hi!

While rebuilding the python related packages against the Python 3.15rc2
version we found that psygnal fails to build from source [1].

The error is: error: Argument 1 to "_fmt_locals" has incompatible type
"FrameLocalsProxyType | dict[str, Any]"; expected "dict[Any, Any]"
[arg-type]

The fix is to widen `_fmt_locals()` to `Mapping`

I've applied this in the sandbox [2] to verify that it builds
successfully, please consider applying the patch to support the upcoming
3.15 version.

Setting the severity to important for now. Once Python 3.15 is released,
it will be added to python3-defaults and this bug will become release
critical.

Happy hacking,

[1]: https://debusine.debian.net/debian/r-python-python3.15/artifact/4644593/
[2]: https://debusine.debian.net/debian/r-python-python3.15/