gh-152817: Prevent deletion of sqlite3 cursor.row_factory attr, missed from: gh-149738

[stestagg]

gh-149738 prevents deletion of row_factory and text_factory attributes on a sqlite3 connection, this PR applies that fix to cursor objects too, to avoid a similar segfault:

import sqlite3

cur = sqlite3.connect(":memory:").cursor()
del cur.row_factory
cur.execute("select 1").fetchone()

I've piggy-backed off the existing docs change and blurb as that seems to cover this already

• Issue: segfault after deleting row_factory attr of sqlite3 cursor #152817

[stestagg]

@sepehr-rs - You were the author of the original, are you able to take a look? Thanks in advance!