Add comments for setup

setup.py

@@ -1,8 +1,8 @@
 #!/usr/bin/env python
 from setuptools import setup, find_packages
-try:
+try:  # for pip >= 10
     from pip._internal.req import parse_requirements as parse
-except ImportError:
+except ImportError:  # for pip <= 9.0.3
     from pip.req import parse_requirements as parse
 
 requirements = lambda f: [str(i.req) for i in parse(f, session=False)]