Python 3.11 needs `find_spec` implemented in JuliaImporter

[MilesCranmer]

With Python 3.11, JuliaImporter should implement find_spec, and JuliaModuleLoader should implement exec_module. Otherwise, the following warnings appear:

<frozen importlib._bootstrap>:1049: ImportWarning: JuliaImporter.find_spec() not found; falling back to find_module()
<frozen importlib._bootstrap>:673: ImportWarning: JuliaModuleLoader.exec_module() not found; falling back to load_module()

Shouldn't be too bad to implement though.

[MilesCranmer]

Yep, I think this is breaking PyJulia on 3.12 @mkitti. Seems like we need to implement find_spec (https://docs.python.org/3/library/importlib.html#importlib.abc.MetaPathFinder.find_spec) in JuliaImporter as well as exec_module in JuliaModuleLoader.

[MilesCranmer]

Fixed from #538