fix(typing): Resolve mypy==1.11.0 issues in plugin_registry

[dangotbanned]

Fixes

• https://github.com/vega/altair/actions/runs/10020215927/job/27697611769
• https://github.com/vega/altair/actions/runs/10013817103/job/27682236626#step:7:15
• https://github.com/vega/altair/actions/runs/10020215927/job/27697611769

altair/utils/plugin_registry.py:213: error: "PluginType" not callable  [misc]
altair/utils/plugin_registry.py:213: note: Error code "misc" not covered by "type: ignore" comment
Found 1 error in 1 file (checked 369 source files)
Error: Process completed with exit code 1.

Description

The release of Mypy 1.11 introduced stricter typing for functools.partial.
The existing # type: ignore comments were no longer sufficient and correcting everything (whilst not breaking all the derived registries) was more complex than I'd like.
Hopefully this summary is easier to understand than the diff alone.

Fix

The fix is mostly 2 changes:

1. Propagating the return type of plugin functions
2. Switching from assert isinstance(...) to a narrowing function

For 2, I have added a courtesy deprecation.
No current altair code (including tests) used the plugin_type argument.
I don't think PluginRegistry is intended to be public as it doesn't appear in alt.__all__.

The new default has identical behaviour to the previous default
It is also safer as it won't be disabled by optimizations & better adheres to the current typing spec guidance.

[dangotbanned]

As an example of how this could be used.

@runtime_checkable would not be useful as it doesn't check types. Any callable would pass, regardless of the return type.

alt.utils.theme.py

from .plugin_registry import PluginRegistry
from typing import Callable
from typing_extensions import TypeAlias, TypeIs

ThemeType: TypeAlias = Callable[..., dict]

def is_theme_plugin(obj: Callable[..., Any]) -> TypeIs[ThemeType]:
    from inspect import signature
    from typing import get_origin

    sig = signature(obj)
    ret = sig.return_annotation
    return ret is dict or get_origin(ret) is dict

class ThemeRegistry(PluginRegistry[ThemeType, dict]):
    pass

alt.vegalite.v5.theme.py

from typing import Final
from ...utils.theme import ThemeRegistry, is_theme_plugin

ENTRY_POINT_GROUP: Final = "altair.vegalite.v5.theme"
themes = ThemeRegistry(entry_point_group=ENTRY_POINT_GROUP, plugin_type=is_theme_plugin)

[binste]

LGTM, thanks for the fix!