setup: Add classifiers to indicate Pygolang works on all Linux / macOS and Windows

Linux and macOS was working before. And we just added Windows support in
the previous patches.

setup.py

@@ -355,6 +355,11 @@ setup(
         Programming Language :: Python :: 3.11
         Programming Language :: Python :: Implementation :: CPython
         Programming Language :: Python :: Implementation :: PyPy
+        Operating System :: POSIX
+        Operating System :: POSIX :: Linux
+        Operating System :: Unix
+        Operating System :: MacOS
+        Operating System :: Microsoft :: Windows
         Topic :: Software Development :: Interpreters
         Topic :: Software Development :: Libraries :: Python Modules\
     """.splitlines()]