fix: fix socket leak in impersonated_credentials (#1123)

* fix: Fix socket leak in impersonated_credentials impersonated_credentials.Credentials.sign_bytes() created a session that wasn't closed leaking a socket. This fixes the issue by always closing the requests session after a signing request is complete. Fixes #1122

Author: stewartmiles

google/auth/impersonated_credentials.py

@@ -288,9 +288,12 @@ def sign_bytes(self, message):
 
  authed_session = AuthorizedSession(self._source_credentials)
 
- response = authed_session.post(
- url=iam_sign_endpoint, headers=headers, json=body
- )
+ try:
+ response = authed_session.post(
+ url=iam_sign_endpoint, headers=headers, json=body
+ )
+ finally:
+ authed_session.close()
 
  if response.status_code != http_client.OK:
  raise exceptions.TransportError(