exception on `cursor.description` from COPY

[dvarrazzo]

Here is a minimal reproduction case:

import psycopg

conn = psycopg.connect(
    user='postgres',
    host='127.0.0.1',
    password='',
    port=5432,
    dbname=None
)
conn.autocommit = True
with conn.cursor() as cur:
    with cur.copy("COPY (select 'This', 'Is', 'Text') TO STDOUT") as copy:
        for row in copy.rows():
            print(row)
    print('---')
    print(cur.description)
    print('---')

The exception is thrown on line print(cur.description):

Traceback (most recent call last):
  File "/Users/irinatruong/Library/Application Support/JetBrains/PyCharm2020.2/scratches/scratch.py", line 16, in <module>
    print(cur.description)
  File "/Users/irinatruong/.pyenv/versions/pgspecial/lib/python3.9/site-packages/psycopg/cursor.py", line 127, in description
    return [Column(self, i) for i in range(res.nfields)]
  File "/Users/irinatruong/.pyenv/versions/pgspecial/lib/python3.9/site-packages/psycopg/cursor.py", line 127, in <listcomp>
    return [Column(self, i) for i in range(res.nfields)]
  File "/Users/irinatruong/.pyenv/versions/pgspecial/lib/python3.9/site-packages/psycopg/_column.py", line 32, in __init__
    raise e.InterfaceError(f"no name available for column {index}")
psycopg.InterfaceError: no name available for column 0

Originally posted by @j-bennet in dbcli/pgspecial#123 (comment)