core: migrate code to benefit from 3.9 stuff (#401)

for now keeping ruff on 3.8 target version, need to sort out modules as well

my/core/warnings.py

@@ -5,14 +5,16 @@ since who looks at the terminal output?
 E.g. would be nice to propagate the warnings in the UI (it's even a subclass of Exception!)
 '''
 
+from __future__ import annotations
+
 import sys
 import warnings
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING
 
 import click
 
 
-def _colorize(x: str, color: Optional[str] = None) -> str:
+def _colorize(x: str, color: str | None = None) -> str:
     if color is None:
         return x
 
@@ -24,7 +26,7 @@ def _colorize(x: str, color: Optional[str] = None) -> str:
     return click.style(x, fg=color)
 
 
-def _warn(message: str, *args, color: Optional[str] = None, **kwargs) -> None:
+def _warn(message: str, *args, color: str | None = None, **kwargs) -> None:
     stacklevel = kwargs.get('stacklevel', 1)
     kwargs['stacklevel'] = stacklevel + 2  # +1 for this function, +1 for medium/high wrapper
     warnings.warn(_colorize(message, color=color), *args, **kwargs)  # noqa: B028