Add CustomTkinter to GUI libraries section

Added CustomTkinter library to GUI libraries list.

Author: Sakth1

README.md

@@ -558,6 +558,7 @@ _Functional Programming with Python._
 _Libraries for working with graphical user interface applications._
 
 - [curses](https://docs.python.org/3/library/curses.html) - Built-in wrapper for [ncurses](http://www.gnu.org/software/ncurses/) used to create terminal GUI applications.
+- [CustomTkinter](https://customtkinter.tomschimansky.com/) - A modern and customizable python UI-library based on Tkinter.
 - [DearPyGui](https://github.com/RaylockLLC/DearPyGui/) - A Simple GPU accelerated Python GUI framework
 - [enaml](https://github.com/nucleic/enaml) - Creating beautiful user-interfaces with Declarative Syntax like QML.
 - [Flet](https://flet.dev) - Cross-platform GUI framework for building modern apps in pure Python. Run on Windows, macOS, Linux, Android, iOS, and the Web.