[AIRFLOW-4777] Simplify python_requires in setup (#5405)

From Airflow 2.0 we support Python 3.5+, but not yet Python 4.0

setup.py

@@ -439,8 +439,9 @@ def do_setup():
             'Intended Audience :: Developers',
             'Intended Audience :: System Administrators',
             'License :: OSI Approved :: Apache Software License',
-            'Programming Language :: Python :: 2.7',
             'Programming Language :: Python :: 3.5',
+            'Programming Language :: Python :: 3.6',
+            'Programming Language :: Python :: 3.7',
             'Topic :: System :: Monitoring',
         ],
         author='Apache Software Foundation',
@@ -453,7 +454,7 @@ def do_setup():
             'extra_clean': CleanCommand,
             'compile_assets': CompileAssets
         },
-        python_requires='>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*',
+        python_requires='~=3.5',
     )