Separate requirements for prod and dev

[djmgit]

Presently there is a single requirements.txt containing all the dependencies.
In order to run Saberx, flake8, pytest and pytest-cove are not required.
They can be moved to a separate file.

For example, we can have a new file called requirements-dev.txt which will have the
following contents:

-r requirements.txt
flake8
pytest
pytest-cov

After that we can remove those libraries from the original requirements.txt.