Add status code 502 to the urllib retry list

fixes pypa#11843
related pypa#12008

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,