Changing how 3.5 is dropped

Currently rpyc 5.0.0 gets installed on Python 3.5 (IIRC setup.py isn't executed on the installed system for wheels).  This change will tell pip that this version isn't compatible with 3.5.

setup.py

@@ -8,9 +8,6 @@
 except ImportError:
     from distutils.core import setup
 
-if sys.version_info < (3, 6):
-    sys.exit("requires python 3.6 and up")
-
 here = os.path.dirname(__file__)
 exec(open(os.path.join(here, 'rpyc', 'version.py')).read())
 
@@ -43,6 +40,7 @@
       #        ],
       #    ),
       platforms=["POSIX", "Windows"],
+      python_requires='>=3.6',
       use_2to3=False,
       zip_safe=False,
       long_description=open(os.path.join(here, "README.rst"), "r").read(),