Lock python dependencies by adopoting `pip-tools`

Description

This PR introduces the use of pip-tools and locks all dependencies versions for the backend.

[ ] I added tests to new or existing code
[ ] I removed hardcoded strings and used react-i18next library (useTranslation hook and/or Trans component), see an example here
[x] I made sure no sensitive info gets logged at any time in the codebase (see here) (e.g. no user info or details, no stacktraces, etc.)
[x] I made sure that any GitHub issue solved by this PR is correctly linked
[ ] I checked that infrastructure/update_infrastructure.sh runs without any error
[x] I checked that npm run build builds without any error
[x] I checked that clusters are listed correctly
[x] I checked that a new cluster can be created (config is produced and dry run passes)
[x] I checked that login and logout work as expected

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