add _GeneratorContextManagerBase.__init__

[tungol]

I think that #6676 showed that Paramspec didn't work, but that wasn't actually the fault of _GeneratorContextManagerBase.

We'll see what mypy-primer has to say about it.

[tungol]

Ah, well, that's no good then.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pyinstrument (https://github.com/joerick/pyinstrument)
- pyinstrument/vendor/decorator.py:295: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManager[_T_co], Callable[..., Iterator[_T_co]], tuple[Any, ...], dict[str, Any]], None]")  [assignment]
+ pyinstrument/vendor/decorator.py:295: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManagerBase[_G], Callable[..., _G], tuple[Any, ...], dict[str, Any]], None]")  [assignment]
- pyinstrument/vendor/decorator.py:301: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManager[_T_co], Callable[..., Iterator[_T_co]], tuple[Any, ...], dict[str, Any]], None]")  [assignment]
+ pyinstrument/vendor/decorator.py:301: error: Incompatible types in assignment (expression has type "Callable[[Any, Any, VarArg(Any), KwArg(Any)], Any]", variable has type "Callable[[_GeneratorContextManagerBase[_G], Callable[..., _G], tuple[Any, ...], dict[str, Any]], None]")  [assignment]