Hardcode bytea and bytea[] oids to their values in PostgreSQL

These values haven't changed since the types have been introduced to PostgreSQL:

https://archive.softwareheritage.org/browse/revision/d31084e9d1118b25fd16580d9d8c2924b5740dff/?path=src/backend/catalog/pg_type.h#L95
https://archive.softwareheritage.org/browse/revision/d31084e9d1118b25fd16580d9d8c2924b5740dff/?path=src/backend/catalog/pg_type.h#L200

swh/core/db/__init__.py

@@ -74,17 +74,12 @@ class BaseDb:
     def adapt_conn(cls, conn):
         """Makes psycopg2 use 'bytes' to decode bytea instead of
         'memoryview', for this connection."""
-        cur = conn.cursor()
-        cur.execute("SELECT null::bytea, null::bytea[]")
-        bytea_oid = cur.description[0][1]
-        bytea_array_oid = cur.description[1][1]
-
         t_bytes = psycopg2.extensions.new_type(
-            (bytea_oid,), "bytea", typecast_bytea)
+            (17,), "bytea", typecast_bytea)
         psycopg2.extensions.register_type(t_bytes, conn)
 
         t_bytes_array = psycopg2.extensions.new_array_type(
-            (bytea_array_oid,), "bytea[]", t_bytes)
+            (1001,), "bytea[]", t_bytes)
         psycopg2.extensions.register_type(t_bytes_array, conn)
 
     @classmethod