Raise UnsupportedFormat for a list with non-uniform row types in _set_dict

[eeshsaxena]

Hit this loading some YAML that came back from an external source. A list whose first item is a mapping but which contains a later non-mapping item, e.g.

- {id: 0, name: n0}
- {id: 1, name: n1}
-

(that trailing bare - parses to None) raises AttributeError: 'NoneType' object has no attribute 'values' from Dataset._set_dict instead of a tablib error.

_set_dict decides the shape from pickle[0] alone and then assumes every row has that type: the dict branch does list(row.values()) on each row, and the list-of-rows branch does Row(row), so a later element of a different type raises a bare AttributeError (or TypeError for the list branch) rather than the UnsupportedFormat that malformed input normally gets. I added a per-row type check to both branches.

Added a test next to the existing YAML import tests; it fails on master with the AttributeError and passes with the change, and the dict/yaml/json/csv tests still pass. Found it while fuzzing the format importers with mutated input.

[eeshsaxena]

Hi! Gentle nudge on this one whenever you have some bandwidth. It's a small, self-contained fix (Raise UnsupportedFormat for a list with non-uniform row types in _set_dict), and it's currently mergeable with no conflicts. No urgency at all, and I'm happy to make any changes you'd like. Thanks for maintaining tablib!