Warn on Python 2.7

Author: dstufft

news/6148.removal

@@ -0,0 +1,2 @@
+Start printing a warning for Python 2.7 to warn of impending Python 2.7 End-of-life and
+prompt users to start migrating to Python 3.

src/pip/_internal/cli/base_command.py

@@ -143,6 +143,14 @@ def main(self, args):
                 replacement=None,
                 gone_in='19.2',
             )
+        elif sys.version_info[:2] == (2, 7):
+            deprecated(
+                "Python 2.7 will reach the end of it's life on January 1st, 2020. "
+                "Please upgrade your Python as Python 2.7 won't be maintained after "
+                "that date. A future version of pip will drop support for Python 2.7.",
+                replacement=None,
+                gone_in=None,
+            )
 
         # TODO: Try to get these passing down from the command?
         #       without resorting to os.environ to hold these.