Set min python version to 3.6

This is already the case since f-strings are used.

setup.py

@@ -30,13 +30,9 @@ setup(
         'Environment :: Console',
         'License :: OSI Approved :: GNU General Public License v3 (GPLv3)',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.4',
-        'Programming Language :: Python :: 3.5',
-        'Programming Language :: Python :: 3.6',
-        'Programming Language :: Python :: 3.7',
     ],
     packages=['toot', 'toot.tui'],
-    python_requires=">=3.4",
+    python_requires=">=3.6",
     install_requires=[
         "requests>=2.13,<3.0",
         "beautifulsoup4>=4.5.0,<5.0",