PR: Fixes to make the app work with PySide6

[ccordoba12]

Description of Changes

• This PR fixes many crashes and errors with PySide6. Now Spyder starts and seems mostly functional with it (but I only tested it on Linux).
• There are still some minor UI issues (e.g. with comboboxes), but those can be fixed in a follow-up PR.

Issue(s) Resolved

Fixes #

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: @ccordoba12

[rear1019]

@ccordoba12, how should we handle patches by me? I already have some 😃

• I could provide patch files.
• You could add me as a collaborator to your fork of Spyder.
• Or some other option.

[ccordoba12]

how should we handle patches by me? I already have some 😃

Could you create a PR against this PR and open it in my fork? I think that's possible and in that way it'd be easy for me to review your work.