Update service.py

[vinraspa]

Add datetime.datetime: "datetime" to ODS_WRITE_FORMAT_COVERSION dict.
This pull request is needed to add datetime.datetime support to pyexcel_ods3 module (pyexcel_ods3/odsw.py).
Thank you

With your PR, here is a check list:

• Has test cases written?
• Has all code lines tested?
• Has make format been run?
• Please update CHANGELOG.yml(not CHANGELOG.rst)
• Has fair amount of documentation if your change is complex
• Agree on NEW BSD License for your contribution

[codecov-commenter]

Codecov Report

Merging #109 (1b4fde5) into dev (67c111f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #109   +/-   ##
=======================================
  Coverage   97.91%   97.91%           
=======================================
  Files          52       52           
  Lines        3361     3361           
=======================================
  Hits         3291     3291           
  Misses         70       70           

Impacted Files	Coverage Δ
pyexcel_io/service.py	97.39% <ø> (ø)

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