Fix entry point discovery on Windows. (#572)

The zipfile module always uses forward slashes, so we shouldn't use
os.path.join here.

Author: phst

python/pip_install/extract_wheels/lib/wheel.py

@@ -55,9 +55,12 @@ def entry_points(self) -> Dict[str, str]:
             # Calculate the location of the entry_points.txt file
             metadata = self.metadata
             name = "{}-{}".format(metadata.name.replace("-", "_"), metadata.version)
-            entry_points_path = os.path.join(
-                "{}.dist-info".format(name), "entry_points.txt"
-            )
+            # Note that the zipfile module always uses the forward slash as
+            # directory separator, even on Windows, so don't use os.path.join
+            # here.  Reference for Python 3.10:
+            # https://github.com/python/cpython/blob/3.10/Lib/zipfile.py#L355.
+            # TODO: use zipfile.Path once 3.8 is our minimum supported version
+            entry_points_path = "{}.dist-info/entry_points.txt".format(name)
 
             # If this file does not exist in the wheel, there are no entry points
             if entry_points_path not in whl.namelist():