[3.14] gh-134262: Add retries to downloads in PCbuild\get_external.py (GH-134865)

(cherry picked from commit e9d845b41d)

Includes fix for off-by-one error from GH-134867
(cherry-picked from commit e64395e8eb)

Co-authored-by: Emma Smith <emma@emmatyping.dev>

PCbuild/get_external.py

@@ -9,6 +9,25 @@ import zipfile
 from urllib.request import urlretrieve
 
 
+def retrieve_with_retries(download_location, output_path, reporthook,
+                          max_retries=7):
+    """Download a file with exponential backoff retry and save to disk."""
+    for attempt in range(max_retries + 1):
+        try:
+            resp = urlretrieve(
+                download_location,
+                output_path,
+                reporthook=reporthook,
+            )
+        except ConnectionError as ex:
+            if attempt == max_retries:
+                msg = f"Download from {download_location} failed."
+                raise OSError(msg) from ex
+            time.sleep(2.25**attempt)
+        else:
+            return resp
+
+
 def fetch_zip(commit_hash, zip_dir, *, org='python', binary=False, verbose):
     repo = f'cpython-{"bin" if binary else "source"}-deps'
     url = f'https://github.com/{org}/{repo}/archive/{commit_hash}.zip'
@@ -16,10 +35,10 @@ def fetch_zip(commit_hash, zip_dir, *, org='python', binary=False, verbose):
     if verbose:
         reporthook = print
     zip_dir.mkdir(parents=True, exist_ok=True)
-    filename, headers = urlretrieve(
+    filename, _headers = retrieve_with_retries(
         url,
         zip_dir / f'{commit_hash}.zip',
-        reporthook=reporthook,
+        reporthook
     )
     return filename