BUG: Fix inconsistent datetime types between calamine and openpyxl readers (#59186)

The calamine Excel reader was returning pd.Timestamp and pd.Timedelta
for datetime values in mixed-type columns, while openpyxl returns
standard library datetime.datetime and datetime.timedelta objects.

This change modifies the calamine reader to return standard library
datetime types, ensuring consistent behavior across Excel reader engines.

- Return datetime.datetime as-is instead of converting to pd.Timestamp
- Convert date objects to datetime to match openpyxl behavior
- Return datetime.timedelta as-is instead of converting to pd.Timedelta
- Remove unused pandas import
- Update test helper get_exp_unit() to reflect new consistent behavior

Author: yjennyli

pandas/io/excel/_calamine.py

@@ -15,7 +15,6 @@
 from pandas.compat._optional import import_optional_dependency
 from pandas.util._decorators import doc
 
-import pandas as pd
 from pandas.core.shared_docs import _shared_docs
 
 from pandas.io.excel._base import BaseExcelReader
@@ -107,10 +106,16 @@ def _convert_cell(value: _CellValue) -> Scalar | NaTType | time:
                     return val
                 else:
                     return value
+            elif isinstance(value, datetime):
+                # Return datetime.datetime as-is to match openpyxl behavior (GH#59186)
+                return value
             elif isinstance(value, date):
-                return pd.Timestamp(value)
+                # Convert date to datetime to match openpyxl behavior (GH#59186)
+                return datetime(value.year, value.month, value.day)
             elif isinstance(value, timedelta):
-                return pd.Timedelta(value)
+                # Return datetime.timedelta as-is to match openpyxl behavior (GH#59186)
+                # (previously returned pd.Timedelta)
+                return value
             elif isinstance(value, time):
                 return value
 

pandas/tests/io/excel/test_readers.py

@@ -135,12 +135,7 @@ def df_ref(datapath):
 
 
 def get_exp_unit(read_ext: str, engine: str | None) -> str:
-    unit = "us"
-    if read_ext == ".ods" and engine == "odf":
-        pass
-    elif (read_ext == ".ods") ^ (engine == "calamine"):
-        unit = "s"
-    return unit
+    return "us"
 
 
 def adjust_expected(expected: DataFrame, read_ext: str, engine: str | None) -> None: