refactor: replace regex with str.startswith and add raw-file URL mapping

Per review suggestion from hugovk:
- Replace re.match with str.startswith for clarity (re.match anchors start
  but not end, which can be confusing)
- Also handle hg.python.org/cpython/raw-file/ URLs, mapping them to
  raw.githubusercontent.com (e.g. Python 2.7.3 release notes use this form)

Author: nagasrisai

apps/downloads/models.py

@@ -123,18 +123,29 @@ def corrected_release_notes_url(self):
         the "Release notes" links on the downloads page still work for legacy
         releases (3.3.6 and earlier).
 
-        Example::
+        Examples::
 
             http://hg.python.org/cpython/file/v3.3.6/Misc/NEWS
             → https://github.com/python/cpython/blob/v3.3.6/Misc/NEWS
+
+            http://hg.python.org/cpython/raw-file/v2.7.3/Misc/NEWS
+            → https://raw.githubusercontent.com/python/cpython/v2.7.3/Misc/NEWS
         """
         url = self.release_notes_url
         if not url:
             return url
-        match = re.match(r"https?://hg\.python\.org/cpython/file/([^/]+)/(.+)", url)
-        if match:
-            tag, path = match.group(1), match.group(2)
-            return f"https://github.com/python/cpython/blob/{tag}/{path}"
+        for prefix in (
+            "http://hg.python.org/cpython/file/",
+            "https://hg.python.org/cpython/file/",
+        ):
+            if url.startswith(prefix):
+                return "https://github.com/python/cpython/blob/" + url[len(prefix):]
+        for prefix in (
+            "http://hg.python.org/cpython/raw-file/",
+            "https://hg.python.org/cpython/raw-file/",
+        ):
+            if url.startswith(prefix):
+                return "https://raw.githubusercontent.com/python/cpython/" + url[len(prefix):]
         return url
 
     def download_file_for_os(self, os_slug):