Update most modules in backend/requirements.txt to latest version, set target to python 3.6

[talavis]

Describe the pull request:

• [ ] Bug fix
• [X] Functional change
• [ ] New feature
• [ ] Code cleanup
• [X] Build system change
• [ ] Documentation change
• [ ] Language translation

Pull request long description:

Update all required backend modules to the latest version with exception of peewee (which would require major changes).

Also set the target Python version to 3.6 for travis.