Readded pygame-gui requirement

setup.py

@@ -10,7 +10,7 @@ with open("README.md") as readme_file:
 with open("CHANGELOG.md") as history_file:
     history = history_file.read()
 
-requirements = ["numpy", "pygame", "scipy", "pytest>=3", "pyyaml"]
+requirements = ["numpy", "pygame", "scipy", "pytest>=3", "pyyaml", "pygame-gui"]
 
 test_requirements = [
     "pytest>=3",