Correct and complete type annotations in Python bindings

[dhdaines]

The type annotations in the Python bindings were quite incorrect in a number of places.

In various other places they were simply missing!

Also you should not use from __future__ import annotations, it was a misfeature that is deprecated and will be removed soon, see: https://docs.python.org/3/library/__future__.html#id2

This also revealed what appear to be a number of coding errors. In particular you don't need to use ctypes.pointer in most places where you can just use ctypes.byref. I have removed this except in the one case where it is absolutely necessary (in VectorFst.to_bytes where we have to pass a reference to a pointer, because the pointer itself is getting replaced)

Also the various arguments to Tr.__init__ really should absolutely never be None except in the curious case where one is being created from an existing pointer.

It would probably be better to use Tr.__new__ or create a separate class method to reference an existing underlying pointer (also ... how does this interact with gc?) but I have kept the interface the same for now, just added the proper overload declarations and ensured that the code actually does what they say.

Also MutableTrsIterator isn't a real Python iterator and probably shouldn't define __iter__ and __next__ if it isn't going to behave like one, but again, that would be a different change.