Double-escape paths on Windows

This helps ensure that they aren't improperly handled due to the newer
string-in-string design for the setuptools invocation script.

src/pip/_internal/utils/setuptools_build.py

@@ -3,6 +3,8 @@ import textwrap
 from typing import List, Optional, Sequence
 
 # Shim to wrap setup.py invocation with setuptools
+# Note that __file__ is handled via two {!r} *and* %r, to ensure that paths on
+# Windows are correctly handled (it should be "C:\\Users" not "C:\Users").
 _SETUPTOOLS_SHIM = textwrap.dedent(
     """
     exec(compile('''
@@ -27,7 +29,7 @@ _SETUPTOOLS_SHIM = textwrap.dedent(
         )
         sys.exit(1)
 
-    __file__ = {!r}
+    __file__ = %r
     sys.argv[0] = __file__
 
     if os.path.exists(__file__):
@@ -39,7 +41,7 @@ _SETUPTOOLS_SHIM = textwrap.dedent(
         setup_py_code = "from setuptools import setup; setup()"
 
     exec(compile(setup_py_code, filename, "exec"))
-    ''', "<pip-setuptools-caller>", "exec"))
+    ''' % ({!r},), "<pip-setuptools-caller>", "exec"))
     """
 ).rstrip()
 

tests/unit/test_utils.py

@@ -948,7 +948,7 @@ def test_make_setuptools_shim_args() -> None:
     shim = args[3]
     # Spot-check key aspects of the command string.
     assert "import setuptools" in shim
-    assert "__file__ = '/dir/path/setup.py'" in args[3]
+    assert "'/dir/path/setup.py'" in args[3]
     assert "sys.argv[0] = __file__" in args[3]