Add obnoxious warning about Python 2 being unsupported on this release with...

Add obnoxious warning about Python 2 being unsupported on this release with guidance on how to avoid the warning and what to do if that guidance was ineffective.

pkg_resources/__init__.py

@@ -83,6 +83,7 @@ __import__('pkg_resources.extern.packaging.version')
 __import__('pkg_resources.extern.packaging.specifiers')
 __import__('pkg_resources.extern.packaging.requirements')
 __import__('pkg_resources.extern.packaging.markers')
+__import__('pkg_resources.py2_warn')
 
 
 __metaclass__ = type

pkg_resources/py2_warn.py

+import sys
+import warnings
+import textwrap
+
+
+msg = textwrap.dedent("""
+    You are running Setuptools on Python 2, which is no longer
+    supported and
+    >>> SETUPTOOLS WILL STOP WORKING <<<
+    in a subsequent release. Please ensure you are installing
+    Setuptools using pip 9.x or later or pin to `setuptools<45`
+    in your environment.
+    If you have done those things and are still encountering
+    this message, please comment in
+    https://github.com/pypa/setuptools/issues/1458
+    about the steps that led to this unsupported combination.
+    """)
+
+sys.version_info < (3,) and warnings.warn("*" * 60 + msg + "*" * 60)