update CITATION.cff to 1.2.0

[sappelhoff]

• update CITATION.cff to 1.2.0
• add some fields that are now supported
• tweak fields: message ("below" -> "in this file"), url (now point to readthedocs)
• turn some fields into explicit strings, to avoid potential YAML issues.
• add YAML version and the optional --- and ... signifiers for start and end of the document

and also:

• bump flake8 and flake8-isort versions because somehow they were incompatible in their current combination

[codecov[bot]]

Codecov Report

Merging #81 (7ffaa6e) into main (ee1c789) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #81   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          423       423           
=========================================
  Hits           423       423           

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