Fix return type of parse_isodatetime

parse_isodatetime had a return type that included None, inherited
from an earlier version of the function, but it raises an exception
if the provided string is not valid and can never return None. Fix
the return type to match.

changelog.d/20240219_113514_rra_DM_42937.md

@@ -0,0 +1,3 @@
+### Bug fixes
+
+- Fix the return type of `safir.datetime.parse_isodatetime` to not include `None` since the function never returns `None`.

src/safir/datetime.py

@@ -111,14 +111,14 @@ def isodatetime(timestamp: datetime) -> str:
     return timestamp.strftime("%Y-%m-%dT%H:%M:%SZ")
 
 
-def parse_isodatetime(time_string: str) -> datetime | None:
+def parse_isodatetime(time_string: str) -> datetime:
     """Parse a string in a standard ISO date format.
 
     Parameters
     ----------
     time_string
         Date and time formatted as an ISO 8601 date and time using ``Z`` as
-        the time zone.  This is the same format produced by `isodatetime` and
+        the time zone. This is the same format produced by `isodatetime` and
         is compatible with Kubernetes and the IVOA UWS standard.
 
     Returns
@@ -134,8 +134,9 @@ def parse_isodatetime(time_string: str) -> datetime | None:
     Notes
     -----
     When parsing input for a model, use `safir.pydantic.normalize_isodatetime`
-    instead of this function.  Using a model will be the normal case; this
-    function is primarily useful in tests.
+    instead of this function. Using a model will be the normal case; this
+    function is primarily useful in tests or for the special parsing cases
+    required by the IVOA UWS standard.
     """
     if not time_string.endswith("Z"):
         raise ValueError(f"{time_string} does not end with Z")