Verify that passed intervals are lists of lists

Trying to second guess the intent of the caller made the types and
implementation more complex, and had a bug: if `intervals[0]` was `None`
(that is, the user had passed a single interval with an open start)
rather than a `datetime` then `intervals` would wrongly be treated as a
list of lists.

pystac/collection.py

@@ -151,16 +151,10 @@ class TemporalExtent:
         Datetimes are required to be in UTC.
     """
 
-    def __init__(
-        self, intervals: Union[List[List[Optional[Datetime]]], List[Optional[Datetime]]]
-    ):
-        # A common mistake is to pass in a single interval instead of a
-        # list of intervals. Account for this by transforming the input
-        # in that case.
-        if isinstance(intervals, list) and isinstance(intervals[0], Datetime):
-            self.intervals = [cast(List[Optional[Datetime]], intervals)]
-        else:
-            self.intervals = cast(List[List[Optional[Datetime]]], intervals)
+    def __init__(self, intervals: List[List[Optional[Datetime]]]):
+        assert isinstance(intervals, list)
+        assert isinstance(intervals[0], list)
+        self.intervals = intervals
 
     def to_dict(self) -> Dict[str, Any]:
         """Generate a dictionary representing the JSON of this TemporalExtent.