Deprecate support for Python < 3.6 (#5985)

* Deprecate support for Python < 3.6

This is a first step, beyond this we can:
* remove all from future imports
* remove 'six' lib as a dependency
* start using f-strings
* enjoy ourselves

* fix tox

* Rebasing

* fix

setup.py

@@ -8,9 +8,13 @@ import io
 import json
 import os
 import subprocess
+import sys
 
 from setuptools import find_packages, setup
 
+if sys.version_info < (3, 6):
+    sys.exit('Sorry, Python < 3.6 is not supported')
+
 BASE_DIR = os.path.abspath(os.path.dirname(__file__))
 PACKAGE_DIR = os.path.join(BASE_DIR, 'superset', 'static', 'assets')
 PACKAGE_FILE = os.path.join(PACKAGE_DIR, 'package.json')
@@ -109,8 +113,6 @@ setup(
         '/apache/incubator-superset/tarball/' + version_string
     ),
     classifiers=[
-        'Programming Language :: Python :: 2.7',
-        'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
     ],
 )