[3.10] pythongh-79579: Improve DML query detection in sqlite3 (pythonGH-93623)

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 4674007)

Co-authored-by: Erlend Egeberg Aasland <[email protected]>

Author: erlend-aasland