Set python_requires in setup.py to ">= 3.4"

In addition to replacing the manual version check, this will also
instruct pip to download an older version of mwparserfromhell for
users running earlier Python versions rather than just getting something
broken.

setup.py

@@ -25,9 +25,6 @@ from glob import glob
 from os import environ
 import sys
 
-if sys.version_info[1] == 3 and sys.version_info[1] < 4:
-    raise RuntimeError("mwparserfromhell needs 3.4+")
-
 from setuptools import setup, find_packages, Extension
 from setuptools.command.build_ext import build_ext
 
@@ -80,6 +77,7 @@ setup(
     ext_modules = [tokenizer] if use_extension else [],
     test_suite = "tests",
     version = __version__,
+    python_requires = ">= 3.4",
     author = "Ben Kurtovic",
     author_email = "ben.kurtovic@gmail.com",
     url = "https://github.com/earwig/mwparserfromhell",