Clearly document that exceptions shouldn't import any internals
This file should be importable in all other modules, which means it
can't be importing any of those modules (to prevent an import loop).
-"""Exceptions used throughout package"""
+"""Exceptions used throughout package.
+
+This module MUST NOT try to import from anything within `pip._internal` to
+operate. This is expected to be importable from any/all files within the
+subpackage and, thus, should not depend on them.
+"""
import configparser
import re
