ExtensionFileLoader has invalid stubs

[Holt59]

These ExtensionFileLoader is missing proper __init__:

 class importlib.machinery.ExtensionFileLoader(fullname, path)

https://docs.python.org/3/library/importlib.html#importlib.machinery.ExtensionFileLoader

[srittau]

And several other methods and fields it seems. Any PRs welcome!

[hardikpnsp]

The __init__ method was added as part of #5148, but several other methods in ExtensionFileLoader are missing. I can take that up.