fix: alias datetime import to satisfy ruff ICN001

Repo-wide ruff (uvx, latest) flags maths/zellers_congruence.py:2
ICN001 and this job blocks unrelated open PRs. One usage site was
renamed accordingly; doctests unchanged.

Author: peda-cos

maths/zellers_congruence.py

@@ -1,5 +1,5 @@
 import argparse
-import datetime
+import datetime as dt
 
 
 def zeller(date_input: str) -> str:
@@ -120,7 +120,7 @@ def zeller(date_input: str) -> str:
         )
 
     # Get datetime obj for validation
-    dt_ck = datetime.date(int(y), int(m), int(d))
+    dt_ck = dt.date(int(y), int(m), int(d))
 
     # Start math
     if m <= 2: