Revert

vendor/pip-pop/pip-diff

@@ -12,13 +12,9 @@ Options:
 """
 import os
 from docopt import docopt
-from requests import session
-try:  # for pip >= 10
-    from pip._internal.index import PackageFinder
-    from pip._internal.req import parse_requirements
-except ImportError:  # for pip <= 9.0.3
-    from pip.index import PackageFinder
-    from pip.req import parse_requirements
+from pip.req import parse_requirements
+from pip.index import PackageFinder
+from pip._vendor.requests import session
 
 requests = session()