[Python-server] Fix blueplanet 'file not found' error

Created by: sebastien-rosset

Fix the following build issue:

Running ./bin/python-server-blueplanet-petstore.sh (output to /dev/null)
--
4699 | rm: cannot remove 'samples/server/petstore/python-blueplanet/test-requirements.txt': No such file or directory

This PR build fails because of unrelated build issue in Java-jersey2. That build issue should be fixed with #6413

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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