Thx to Jonrshape.
He commented on my question with the solution, that is documented in the python docs.

To use the ISO calendar values for the week and weekday, one has to use

• %G - ISO 8601 year

• %V - ISO 8601 week as a decimal number with Monday as the first day of the week.

• %u - ISO 8601 weekday as a decimal number where 1 is Monday.

instead of "%Y %W %w"