Switch setup to Poetry

[adrhill]

This PR updates the packaging tools, dev dependencies and CI. The source code remains untouched.

Relevant requirements

• [SWR 1.1.3]: Can be installed via pip.
• [SWR 3.1.1]: Use and enforcement of codings style.
• [SWR 3.1.2]: Use of automatic CI pipeline.

---

Changes

Project structure

• switch setup to Poetry for packaging and dependency management
  • updated release guide todo_before_release.md
• use GitHub Actions instead of Travis CI
  • add code style checks with black and isort
  • track code coverage with codecov
• update project structure
  • use separate tests folder
  • src layout
• setup dev dependencies, e.g.:
  • pytest config in pyproject.toml
  • isort config in pyproject.toml
  • flake8 config in .flake8
  • pylint config in .pylintrc
• setup tox in tox.ini
  • run pytest, isort, black, flake8, pylint and try to build docs

Breaking changes

• remove DeepLIFT
• update lower bound of dependencies to Python 3.7, TensorFlow 1.15 and Keras 2.3

Currently, I've tagged this as 1.10.0 in the pyproject.toml for the next release, however you might want to use version number 2.0.0 according to the Semantic Versioning convention, as this makes incompatible API changes by removing DeepLIFT.

Reasoning

Poetry

Poetry is a dependency manager and packaging tool that uses PEP 518's pyproject.toml to define dependencies, settings and build systems in a single file.

Poetry automatically resolves dependency conflicts within the specified version constraints and makes it trivial to set up venvs using poetry install. Poetry's poetry.lock ensures everyone working on the package uses consistent package versions.

Building and publishing to PyPI is as easy as

poetry build
poetry publish

Command line dev dependencies like pytest, black and tox can be run using poetry run pytest.

GitHub Actions instead of Travis CI

Travis CI is not free of charge anymore, only giving a limited amount of credits to OSS projects.

Project structure

Using a separate tests folders is considered best pratice in the pytest docs. It is "strongly suggested" to use a src layout so that tools like tox test installed versions of the package instead of local code from the repository.

More discussion can be found here and here.

Dependencies

This PR temporarily uses Keras 2.3 as it is the first and last multi-backend Keras release that supports both TF2 as well as TF1.

[albermax]

What is the reason to remove deeplift?

Mind that there are merge conflicts that need to fixed first (see beow).

[adrhill]

Sorry, that rebase didn't work quite as I imagined it... Let me reopen the PR to get a clean Git commit history.