Array of tuples as a parameter, with type casts

I have a query like this:

query = '''
insert into t
select * from unnest (%s)
as t1(c1 timestamp, c2 int)
'''

The reason of the approach is obviously reducing the number of server
roundtrips when inserting many rows.
Usage:

from datetime import datetime
import psycopg2
db =
psycopg2.connect('postgres://postgres:******(at)host/postgres?sslmode=prefer')
cur = db.cursor()
cur.execute(query, ([(datetime.now(), 1), (datetime.now(), 2)],))
db.commit()
db.close()

Recently they needed to extend the column set by a text and a bigint
columns. Neither of them works:

from datetime import datetime
import psycopg2
query = '''insert into t
select * from unnest (%s)
as t1(c1 timestamp, c2 int, c3 text, c4 bigint)
'''
db = psycopg2.connect('postgres://postgres(at)localhost/postgres')
cur = db.cursor()
cur.execute(query, ([(datetime.now(), 1, 'abc', 100), (datetime.now(), 2,
'xyz', 200)],))
db.commit()
db.close()

It throws:
psycopg2.errors.DatatypeMismatch: function return row and query-specified
return row do not match
DETAIL: Returned type unknown at ordinal position 3, but query expects
text.

The problem is that the library sends the following request to the DBMS:

insert into t
select * from unnest (ARRAY[('2019-11-29T19:48:19.683310'::timestamp, 1,
'abc', 100),('2019-11-29T19:48:19.683320'::timestamp, 2, 'xyz', 200)])
as t1(c1 timestamp, c2 int, c3 text, c4 bigint)

For whatever reason it fails with the above error, but OK, it is related to
Postgres, not to psycopg2.
I can make it work by specifying type casts for text and bigint columns:

insert into t
select * from unnest (ARRAY[('2019-11-29T19:48:19.683310'::timestamp, 1,
'abc'::text, 100::bigint),('2019-11-29T19:48:19.683320'::timestamp, 2,
'xyz'::text, 200::bigint)])
as t1(c1 timestamp, c2 int, c3 text, c4 bigint);

But I need a way to make the psycopg2 module to do that. Another workaround
would be creation a row type for the desired set of columns and casting %s
to this type[], but I would not like to create additional objects in the
database because it is pretty much out of my control.

Your help is very appreciated.

P.S. I am aware of other solutions such as execute_batch(),
execute_values(), etc. Take this question as a theoretical one, I just want
to understand if user is able to control this particular aspect of the
module.

psycopg2-binary==2.8.4
Postgres ~ any