Fix datetime imports

[pnuu]

As discussed in #175 , it is better to use import datetime as dt instead of from datetime import datetime so that the module name does not replace the package preventing usage of previously unimported modules. This PR fixes the imports.

[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 12 lines in your changes missing coverage. Please review.

Project coverage is 87.83%. Comparing base (16f71f7) to head (3384cf8).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
pyorbital/orbital.py	46.66%	8 Missing ⚠️
pyorbital/geoloc.py	0.00%	2 Missing ⚠️
pyorbital/geoloc_example.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #177   +/-   ##
=======================================
  Coverage   87.83%   87.83%           
=======================================
  Files          16       16           
  Lines        2630     2630           
=======================================
  Hits         2310     2310           
  Misses        320      320           

Flag	Coverage Δ
unittests	87.83% <78.94%> (ø)

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[coveralls]

coverage: 87.798%. remained the same
when pulling 3384cf8 on pnuu:fix-datetime-imports
into 932c493 on pytroll:main.