fix python_requires= to allow installation under py3.5

According to PEP440, a comma in a version specifier behaves as a logical AND,
so the previous "~=3.5,~=3.6" is equivalent to just "~=3.6", which excludes
python3.5.

This patch replaces it with "~=3.5", which is equivalent to ">=3.5, ==3.*",
so it includes 3.5, 3.6, 3.7, and beyond (but not 4.0).

setup.py

@@ -36,5 +36,5 @@ setup(
     keywords='cryptography noiseprotocol noise security',
     packages=find_packages(exclude=['contrib', 'docs', 'tests', 'examples']),
     install_requires=['cryptography==2.1.4'],
-    python_requires='~=3.5,~=3.6',
+    python_requires='~=3.5', # we like 3.5, 3.6, and beyond, but not 4.0
 )