More consistent object paths between host and target when unpickling

[rth]

In pyodide/pyodide#3202 (comment) @hoodmane raised a good question,

How about changing pytest_pyodide to monkey patch JsException? I think this
change looks weird. If we're going to have a class with the same name both
inside and outside selenium, it should have the same path.

following #44 by @koenvo

Maybe more generally we could just install pyodide-py on the host, that way all those paths would already exist and we don't need to redefine or monkeypatch anything? I just uploaded it to PyPI, still need to automate the release.

This would be more general and should allow unpickling of other objects defined in this package if necessary.

The difficulty there is to make sure we install the pyodide-py version that matches the version of Pyodide used.

[koenvo]

Totally agree with @hoodmane! This would also remove the need for the find_class in Unpickler.

I believe the "versions should match" argument applies to all packages installed, right?