Add status code 502 to the urllib retry list

fixes pypa#11843
related pypa#12008

Author: bdraco

news/11843.feature.rst

@@ -0,0 +1 @@
+Retry on HTTP status code 502

src/pip/_internal/network/session.py

@@ -355,8 +355,9 @@ def __init__(
             # is typically considered a transient error so we'll go ahead and
             # retry it.
             # A 500 may indicate transient error in Amazon S3
+            # A 502 may be a transient error from a CDN
             # A 520 or 527 - may indicate transient error in CloudFlare
-            status_forcelist=[500, 503, 520, 527],
+            status_forcelist=[500, 502, 503, 520, 527],
             # Add a small amount of back off between failed requests in
             # order to prevent hammering the service.
             backoff_factor=0.25,