Fix sqlite3 cursor and placeholder syntax in get_row_from_db (#895)

[harminius]

Summary

Fixes #895 — clicking a version in the Changes viewer raised:

TypeError: 'sqlite3.Cursor' object does not support the context manager protocol
  at Mergin/diff.py:78 in get_row_from_db

get_row_from_db in Mergin/diff.py was written with psycopg2 conventions even though db_conn is a stdlib sqlite3.Connection:

• with db_conn.cursor() as c: — sqlite3.Cursor is not a context manager (only sqlite3.Connection is).
• '"{}" = %s' placeholders — sqlite3 uses ?, not %s.

Changes

• Drop the with wrapper; assign cursor directly
• %s → ? for parameter placeholders.

[varmar05]

@harminius does this mean this was never working?

also I do not see using context manager with connection object - would be good to review gpkg handling here and wrap it up

[harminius]

@varmar05
The bug(s) have been there for two months, as it was introduced with SQL sanitization cd2c4a7

As of context manages

• with works unintuintively - it's transaction-based, not closing the connection https://blog.rtwilson.com/a-python-sqlite3-context-manager-gotcha/
• we can use simple try/finally
• or there is built-in contextlib with closing function if you prefer https://docs.python.org/3/library/contextlib.html

[harminius]

unlike the connection, cursor doesn't lock any OS-level resources
sqlite3 can create the cursor implicitly with the .execute() method

I would prefer not to worry about its lifecycle and use the shortcut

https://docs.python.org/3/library/sqlite3.html#sqlite3-connection-shortcuts

[harminius]

Closing the SQL connection might fix this dinosaur: #473