Include syntax checking on deployments

[garciparedes]

Is your feature request related to a problem? Please describe.
The code quality should be one of the most important things around the development of this project. For that reason, it's necessary to define properly settings over the workflow that help us to validate this objective.

Describe the solution you'd like
Include flake8 as development dependency to validate the proper syntax of the code. This checking should be applied at each Travis build and fail the task if there are any error.

Describe alternatives you've considered
We can implement this checking using another validation tools like Pylint, mypy or pyflakes, but I think that flake8 is a good starting point that can be extended with one (or more) of the described alternative tools.

Additional context
This issue should be implemented through a different branch with its corresponding pull request because it is probably necessary to perform some code improvements.