Improvements to callback handling for select_file(), select_folder(), etc

[hx2A]

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[hx2A]

From #234:

If the callback function has the wrong number of parameters there is a proper error message but I think this can be improved with Python's inspect library to validate the callback when the select method is called.

[hx2A]

And original goal of this issue:

Callbacks for select_file(), select_folder(), etc should be passed Path objects, not strings

[hx2A]

I looked at the code for this and don't think it will be that hard to fix.

[hx2A]

Callbacks for select_file(), select_folder(), etc should be passed Path objects, not strings

@villares I have a fix for this. Now the callbacks will be passed Path objects or None (if the user hits cancel), as originally intended.

Next I will re-evaluate the error handling around this and see if I can make some improvements there.

[hx2A]

If the callback function has the wrong number of parameters there is a proper error message but I think this can be improved with Python's inspect library to validate the callback when the select method is called.

This is done also. Now, if the callback has the wrong number of parameters or is keyword only, py5 will raise an error when py5's select method is called, not when the callback is called. py5 is validating the callback before proceeding with the select method functionality.

One more fix for the error handling: if an exception is thrown in the callback, there is an error message, but no stack trace (at least on my Mac). I'm confused why there is no stack trace. There should be.

[hx2A]

Now stack traces appear when the user's callback has an error. Example below:

py5 encountered an error in your code:

File "/Users/jim/INSTALL/anaconda3/envs/py5/lib/python3.8/site-packages/py5/sketch.py", line 933, in wrapped_callback_py5_no_prune
    return callback(selection if selection is None else Path(selection))
File "test.py", line 5, in f
    4    def f(selection):
--> 5        x = 10 / 0
    6        py5.println(type(selection))

ZeroDivisionError: division by zero

I can't seem to get rid of the wrapped_callback_py5_no_prune bit without also getting rid of the user's code. That's fine though, this is much better than the alternatives.