use concept doi in CITATION.cff

[sverhoeven]

In

python-template/template/CITATION.cff.jinja

Lines 10 to 12 in 977268c

	date-released: 20??-MM-DD
	doi: <insert your DOI here>
	version: "{{ version }}"

The date-release, version and doi are set. I will forget to update those fields I would rather have

identifiers:
  - description: Latest version of software
    type: doi
    value: "10.5281/zenodo.5119389"

On the first release I need to copy the zenodo concept doi to the cff file and then I do not have to change that section of the file anymore.