core: add helper for opening read-only database

my/core/dataset.py

@@ -0,0 +1,9 @@
+from pathlib import Path
+
+# TODO wonder if also need to open without WAL.. test this on read-only directory/db file
+def connect_readonly(db: Path):
+    import dataset # type: ignore
+    # see https://github.com/pudo/dataset/issues/136#issuecomment-128693122
+    import sqlite3
+    creator = lambda: sqlite3.connect(f'file:{db}?mode=ro', uri=True)
+    return dataset.connect('sqlite:///', engine_kwargs={'creator': creator})