general: deprecate my.core.dataset and simplify tox file

my/core/_deprecated/dataset.py

@@ -0,0 +1,12 @@
+from ..common import PathIsh
+from ..sqlite import sqlite_connect_immutable
+
+
+def connect_readonly(db: PathIsh):
+    import dataset  # type: ignore
+
+    # see https://github.com/pudo/dataset/issues/136#issuecomment-128693122
+    # todo not sure if mode=ro has any benefit, but it doesn't work on read-only filesystems
+    # maybe it should autodetect readonly filesystems and apply this? not sure
+    creator = lambda: sqlite_connect_immutable(db)
+    return dataset.connect('sqlite:///', engine_kwargs={'creator': creator})