Fixes #214

Problem

When a GitHub repository is moved/renamed, the GitHub API returns a 3xx redirect response with a Location header pointing to the new repository's API URL. However, the RedirectionException raised by gidgethub doesn't provide access to the response headers, making it impossible for users to programmatically follow redirects.

Solution

• Modified HTTPException base class to accept an optional headers parameter
• Updated decipher_response() function in sansio.py to pass headers to exceptions
• Added Mapping import to support the new headers parameter type

Changes

• gidgethub/__init__.py: Added headers parameter to HTTPException.__init__() and imported Mapping from typing
• gidgethub/sansio.py: Modified decipher_response() to pass headers when raising exceptions

Usage

Users can now access redirect information from RedirectionException:

try:
    # GitHub API call that might redirect
    pass
except RedirectionException as e:
    location = e.headers.get('location')
    if location:
        # Follow the redirect to the new repository location
        pass

Testing

• All existing tests continue to pass
• New functionality maintains backward compatibility
• Headers are stored as empty dict when not provided

This enhancement enables proper handling of repository moves/renames in GitHub API clients built with gidgethub.