Drop support for end-of-life Python 3.6

Author: hartwork

setup.py

@@ -20,13 +20,12 @@
         "Operating System :: OS Independent",
         "Topic :: Software Development",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
         "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3 :: Only",
     ],
-    python_requires='>=3.6',
+    python_requires='>=3.7',
     setup_requires=[
         'setuptools>=38.6.0',  # for long_description_content_type
     ],