Python: Make __str__ work for CompileError's diagnostics field

[tronical]

As seen in #5469, when printing the CompileError struct, the diagnostics are not correctly transformed to a readable representation. While __str__ is implemented on PyDiagnostics, somehow it doesn't transcend into the list that's created from it.

Fixing this would make it easier to quickly discover the root cause for a run-time generated Slint compile error.

[cooliean]

The Demo works, but when packaged with pyinstaller, it doesn't

python 3.10 slint 1.6.0a8
package command : pyinstaller -FD main.py --add-data "appwindow.slint:./"

Traceback (most recent call last):
  File "main.py", line 5, in <module>
  File "slint/__init__.py", line 182, in load_file
slint.CompileError: ('Could not compile appwindow.slint', [<builtins.PyDiagnostic object at 0x109ff1ac0>])
[53112] Failed to execute script 'main' due to unhandled exception!

import slint
components = slint.load_file("appwindow.slint")
main_window = components.MainWindow()
main_window.run()

[tronical]

I think that may be because when passing a relative path to load_file() will it be resolved against the process' working directory. You could try something like this instead: slint.load_file(os.path.join(os.path.dirname(__file__), "appwindow.slint"))