Fix warnings in test_datetime.py

[vyasr]

Description

Contributes to #9999 and #10363.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mroeschke]

LGTM from a pandas-compat side

[codecov]

Codecov Report

Base: 86.58% // Head: 85.76% // Decreases project coverage by -0.81% ⚠️

Coverage data is based on head (5ef3156) compared to base (b6dccb3).
Patch has no changes to coverable lines.

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-23.02   #12381      +/-   ##
================================================
- Coverage         86.58%   85.76%   -0.82%     
================================================
  Files               155      155              
  Lines             24368    24798     +430     
================================================
+ Hits              21098    21269     +171     
- Misses             3270     3529     +259     

Impacted Files	Coverage Δ
python/cudf/cudf/_version.py	1.41% <0.00%> (-98.59%)	⬇️
python/cudf/cudf/core/buffer/spill_manager.py	74.37% <0.00%> (-5.63%)	⬇️
python/cudf/cudf/utils/dtypes.py	77.77% <0.00%> (-1.69%)	⬇️
python/cudf/cudf/options.py	86.11% <0.00%> (-1.59%)	⬇️
python/cudf/cudf/core/single_column_frame.py	94.30% <0.00%> (-1.27%)	⬇️
python/dask_cudf/dask_cudf/io/parquet.py	91.81% <0.00%> (-0.59%)	⬇️
python/cudf/cudf/core/multiindex.py	91.66% <0.00%> (-0.51%)	⬇️
python/cudf/cudf/core/algorithms.py	90.00% <0.00%> (-0.48%)	⬇️
python/cudf/cudf/core/dataframe.py	93.50% <0.00%> (-0.09%)	⬇️
python/cudf/cudf/core/join/_join_helpers.py	98.52% <0.00%> (-0.07%)	⬇️
... and 32 more

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[vyasr]

@gpucibot merge