So, I made a program with Pygame and PySimpleGui, both licensed under LGPL. I didn´t modified them, just imported and used them on my code. Well, I heard somewhere that GPL 2 is not compatible with GPL 3, and the problem is: Pygame, according to this file, is on the version 2.1 of LGPL license, and PySimpleGUI, according to this file, is licensed under LGPL 3.0. I know LGPL is more open than GPL, but can I use them in the same file of code?
If both are libraries - one licensed under LGPLv2.1 and the other licensed under LGPLv3 - and both are used by your software as a library (i.e. your software is a "work that uses the library"), and your software is not a "work based on the library", then the licenses of those libraries do not apply in a viral way to your software, and to each other.
However, any modifications to the LGPL-licensed library, or any "work based on the library" would automatically be licensed under the same license as the library.
Also, any dependency of those libraries need to have a compatible license with the license of those libraries.
In addition, please note that pygame's license header in the comments of source files (like this one) imply that it is licensed under LGPLv2.1-or-later. So, it may be compatible with LGPLv3 anyway.
The file colordict.py states:
# pygame - Python Game Library
# Copyright (C) 2000-2003 Pete Shinners
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Library General Public
# License as published by the Free Software Foundation; either
# version 2 of the License, or (at your option) any later version.
However, the README file does not state this explicitly.