Add 'python_requires' check (#142)

Prevents unsupported Python versions installing the package

Author: mikemahony

setup.py

@@ -29,6 +29,7 @@
     zip_safe=False,
     packages=find_packages(exclude=["tests"]),
     include_package_data=True,
+    python_requires=">=3.5.2",
     install_requires=[
         "aioredis~=1.0",
         "msgpack~=0.5.2",