fix setup.py for pip>=10

see https://stackoverflow.com/a/49867265/107049

Author: Thomas LEVEIL

setup.py

@@ -4,7 +4,10 @@
 from os import environ as env
 import subprocess
 
-from pip.req import parse_requirements
+try: # for pip >= 10
+    from pip._internal.req import parse_requirements
+except ImportError: # for pip <= 9.0.3
+    from pip.req import parse_requirements
 
 requirements = [str(req.req) for req in parse_requirements('requirements.txt', session=False)]
 requirements_plugins = [str(req.req) for req in parse_requirements('requirements-plugins.txt', session=False)]