changed python version to >=3.8

setup.py

@@ -22,7 +22,7 @@ setuptools.setup(
                  "Intended Audience :: Science/Research",
                  "License :: OSI Approved :: MIT License",
                  "Operating System :: OS Independent",
-                 "Programming Language :: Python :: 3.7",
+                 "Programming Language :: Python :: 3.8",
                  "Topic :: Scientific/Engineering :: Physics",
                  "Topic :: Scientific/Engineering :: Astronomy"],
     package_dir={'csst': 'csst'},