[3.11] gh-79579: Improve DML query detection in sqlite3 (GH-93623) (#93800)

The fix involves using pysqlite_check_remaining_sql(), not only to check
for multiple statements, but now also to strip leading comments and
whitespace from SQL statements, so we can improve DML query detection.

pysqlite_check_remaining_sql() is renamed lstrip_sql(), to more
accurately reflect its function, and hardened to handle more SQL comment
corner cases.
(cherry picked from commit 46740073ef32bf83964c39609c7a7a4772c51ce3)

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@protonmail.com>