Remove six dependency in favor pip's vendored six

Allows removing a dependency of pip-tools. The package is already
guaranteed to exist within pip, so might as well use it.

As more and more projects are dropping Python 2 support, they too are
dropping six as a dependency. As more projects drop six, this will
eventually remove the need for projects to pull in six as a transient
dependency.

When pip eventually drops Python 2 support, it will also drop six. When
that day comes, pip-tools can also drop Python 2 support, so the
eventual lack of six should not be an issue.

piptools/_compat/__init__.py

@@ -2,7 +2,7 @@
 # flake8: noqa
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import six
+from pip._vendor import six
 
 from .pip_compat import PIP_VERSION, parse_requirements
 

piptools/repositories/base.py

@@ -4,7 +4,7 @@
 from abc import ABCMeta, abstractmethod
 from contextlib import contextmanager
 
-from six import add_metaclass
+from pip._vendor.six import add_metaclass
 
 
 @add_metaclass(ABCMeta)

piptools/utils.py

@@ -5,12 +5,12 @@
 from collections import OrderedDict
 from itertools import chain
 
-import six
 from click.utils import LazyFile
 from pip._internal.req.constructors import install_req_from_line
 from pip._internal.utils.misc import redact_auth_from_url
 from pip._internal.vcs import is_url
-from six.moves import shlex_quote
+from pip._vendor import six
+from pip._vendor.six.moves import shlex_quote
 
 from .click import style
 

piptools/writer.py

@@ -4,7 +4,7 @@
 import re
 from itertools import chain
 
-import six
+from pip._vendor import six
 
 from .click import unstyle
 from .logging import log

setup.cfg

@@ -32,7 +32,6 @@ packages = find:
 zip_safe = false
 install_requires =
     click >= 7
-    six
     pip >= 20.1
 
 [options.packages.find]

tests/test_utils.py

@@ -4,8 +4,8 @@
 import os
 
 import pytest
-import six
-from six.moves import shlex_quote
+from pip._vendor import six
+from pip._vendor.six.moves import shlex_quote
 
 from piptools.scripts.compile import cli as compile_cli
 from piptools.utils import (