Close Postgres connections on both success and failure paths (#104)

[BWilliams1004]

Closes #104. Two connection leaks fixed:

sql2csv.py's run_sql_script() only called connection.close() on the
success path if the query raised, execution jumped to the except block, and the connection was never closed. Now wrapped in a finally block.
(with a None guard, since psycopg2.connect() itself could fail before
connection is ever assigned, so it always closes, success or failure.

run_pg.py's run_pg() never closed its connection at all. Rewritten to
Use with context managers for both the connection and the cursor, which
close automatically on exit regardless of outcome.

Testing: Verified locally end-to-end for both. Set up an isolated venv for
sql2datasets its dependencies conflicted with the main venv’s pinned
aiobotocore/botocore versions used a separate env-sql2datasets to
avoid that going forward. For each fix, I ran both a real failing query and
a real successful query against the live database, then confirmed via
pg_stat_activity, the connection counts stayed flat afterward in both
cases, no leaked sessions on success or failure.