PYTHON-3017 Properly check for closed KMS connections (#790)

(cherry picked from commit 99aab1b)

Author: ShaneHarvey

pymongo/encryption.py

@@ -130,6 +130,8 @@ def kms_request(self, kms_context):
             conn.sendall(message)
             while kms_context.bytes_needed > 0:
                 data = conn.recv(kms_context.bytes_needed)
+                if not data:
+                    raise OSError('KMS connection closed')
                 kms_context.feed(data)
         finally:
             conn.close()