setup: use python_requires to check version

setup.py

@@ -17,9 +17,6 @@ def finalize_options(self):
     def run(self):
         self.spawn(['pandoc', '-t', 'man', '-o', 'mkosi.1', 'mkosi.md'])
 
-if sys.version_info < (3, 6):
-    sys.exit("Sorry, we need at least Python 3.6.")
-
 
 setup(
     name="mkosi",
@@ -29,6 +26,7 @@ def run(self):
     maintainer="mkosi contributors",
     maintainer_email="systemd-devel@lists.freedesktop.org",
     license="LGPLv2+",
+    python_requires=">=3.6",
     py_modules=["mkosi"],
     cmdclass = { "man": BuildManpage },
     entry_points="""