Fix datetime import alias in Zeller’s congruence (#15547)

* fix(maths): use conventional datetime alias in Zeller's congruence

* from datetime import date

Replaced datetime module alias with direct import for clarity.

---------

Co-authored-by: Christian Clauss <cclauss@me.com>

Author: Miladkhoshdel

maths/zellers_congruence.py

@@ -1,5 +1,5 @@
 import argparse
-import datetime
+from datetime import date
 
 
 def zeller(date_input: str) -> str:
@@ -119,8 +119,8 @@ def zeller(date_input: str) -> str:
             "Year out of range. There has to be some sort of limit...right?"
         )
 
-    # Get datetime obj for validation
-    dt_ck = datetime.date(int(y), int(m), int(d))
+    # Get date obj for validation
+    dt_ck = date(int(y), int(m), int(d))
 
     # Start math
     if m <= 2: