Specify lowest supported Python version

README.rst

@@ -21,6 +21,8 @@ Install with `pip <https://pip.pypa.io/en/stable/>`__:
 
   $ pip install stix2
 
+Note: The library requires Python 3.6+.
+
 Usage
 -----
 

setup.py

@@ -47,6 +47,7 @@ setup(
     ],
     keywords='stix stix2 json cti cyber threat intelligence',
     packages=find_packages(exclude=['*.test', '*.test.*']),
+    python_requires='>=3.6',
     install_requires=[
         'pytz',
         'requests',