Change the required version of Python to 3.7

Author: soimkim

setup.py

@@ -26,10 +26,10 @@
         download_url='https://github.com/fosslight/fosslight_source_scanner',
         classifiers=['License :: OSI Approved :: Apache Software License',
                      "Programming Language :: Python :: 3",
-                     "Programming Language :: Python :: 3.6",
                      "Programming Language :: Python :: 3.7",
                      "Programming Language :: Python :: 3.8",
                      "Programming Language :: Python :: 3.9", ],
+        python_requires=">=3.7",
         install_requires=required,
         extras_require={":python_version>'3.6'": ["scanoss>=0.7.0"],
                         ":python_version<'3.7'": ["dataclasses", "scanoss"]},