New severity "V" field as "severity_en" attribute

It is identical to the regular severity "S", but it is never localized.

Introduced in Postgres-9.6:
https://www.postgresql.org/docs/9.6/static/protocol-error-fields.html

Author: vitaly-burovoy

asyncpg/exceptions/_base.py

@@ -21,6 +21,7 @@ class PostgresMessageMeta(type):
     _message_map = {}
     _field_map = {
         'S': 'severity',
+        'V': 'severity_en',
         'C': 'sqlstate',
         'M': 'message',
         'D': 'detail',