Updated setup's classifiers keyword argument (#735)

Remove declaration for support of Python 3.6, added declaration
of support of Python 3.9

Author: oleksandr-pavlyk

setup.py

@@ -492,8 +492,8 @@ def _get_cmdclass():
     keywords="dpctl",
     classifiers=[
         "Development Status :: 3 - Alpha",
-        "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
     ],
 )