Update setup.py to require Python 3.6+

more-itertools#578

setup.py

@@ -35,7 +35,7 @@ def get_long_description():
     license='MIT',
     packages=['more_itertools'],
     package_data={'more_itertools': ['py.typed', '*.pyi']},
-    python_requires='>=3.5',
+    python_requires='>=3.6',
     url='https://github.com/more-itertools/more-itertools',
     classifiers=[
         'Development Status :: 5 - Production/Stable',