Fix code coverage + add pyproject.toml

[giampaolo]

• code coverage: move from coveralls to codecov because coveralls does not support python + coverage + pyproject.toml
• move config of coverage and isort tools into pyproject.toml; get rid of their respective config files
• fix python 2.7 integration into Makefile

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@715b14d). Click here to learn what that means. Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #592 +/- ## ========================================= Coverage ? 70.03% ========================================= Files ? 9 Lines ? 3614 Branches ? 0 ========================================= Hits ? 2531 Misses ? 1083 Partials ? 0 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Giampaolo+Rodola)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.