fix: use a real Message for HTTPError hdrs in dependency-failure test

mypy typed hdrs as email.message.Message, so passing None failed the
type check. Pass an empty Message() instead; the value is irrelevant
since the tested code only stringifies the error.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: grische

tests/test_init.py

@@ -3,6 +3,7 @@
 
 import argparse
 import contextlib
+import email.message
 import hashlib
 import os
 import urllib.error
@@ -622,7 +623,7 @@ def fake_mirror(requirement, required_by=None):
                     url="https://example.com/missing-dep/",
                     code=404,
                     msg="Not Found",
-                    hdrs=None,
+                    hdrs=email.message.Message(),
                     fp=None,
                 )
             return None