jinja2 how to remove trailing newline

Change your loop to strip whitespace from the top AND bottom of the output (notice extra - at the for loop close):

{% for key, value in querystring.items() -%}
  {{ key }}: '{{ value }}'
{%- endfor %}

In my tests (using https://github.com/abourguignon/jinja2-live-parser), the - must come after the first {%, not before the last to achieve what you’re asking for.

Docs: https://jinja.palletsprojects.com/en/latest/templates/#whitespace-control