fix dt import in series

Author: TrevorBergeron

bigframes/series.py

@@ -81,7 +81,7 @@
 
 if typing.TYPE_CHECKING:
     import bigframes.geopandas.geoseries
-    import bigframes.operations.datetimes as dt
+    import bigframes.operations.datetimes as datetimes
     import bigframes.operations.strings as strings
 
 
@@ -208,8 +208,10 @@ def __init__(
         self._block.session._register_object(self)
 
     @property
-    def dt(self) -> dt.DatetimeMethods:
-        return dt.DatetimeMethods(self)
+    def dt(self) -> datetimes.DatetimeMethods:
+        import bigframes.operations.datetimes as datetimes
+
+        return datetimes.DatetimeMethods(self)
 
     @property
     def dtype(self):