[mypy] fix small folders (#4292)

* add final else-statement

* fix file_transfer

* fix quantum folder

* fix divide_and_conquer-folder

* Update build.yml

* updating DIRECTORY.md

* Update ripple_adder_classic.py

* Update .github/workflows/build.yml

* removed imports from typing

* removed conversion to string

* Revert "removed conversion to string"

This reverts commit 2f7c4731d103f24c73fb98c9a6898525998774c5.

* implemented suggested changes

* Update receive_file.py

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>
Co-authored-by: Christian Clauss <cclauss@me.com>