Fix Flake8 complaints and datetime imports

[pnuu]

This PR fixes flake8 complaints and fixes the datetime imports. This is a precursor work for #64

We can't do from datetime import datetime because it overwrites the module name thus making subsequent imports from datetime impossible.

[codecov]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 79.90%. Comparing base (a28fb72) to head (be3bb77).
Report is 4 commits behind head on main.

❗ Current head be3bb77 differs from pull request most recent head bf2863b. Consider uploading reports for the commit bf2863b to get more accurate results

Files	Patch %	Lines
posttroll/logger.py	0.00%	2 Missing ⚠️
posttroll/__init__.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   79.87%   79.90%   +0.03%     
==========================================
  Files          17       17              
  Lines        2211     2210       -1     
==========================================
  Hits         1766     1766              
+ Misses        445      444       -1     

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[mraspaud]

LGTM, feel free to merge