Switching setup to Poetry

[padix-key]

Currently, Biotite uses the classical setuptools with a setup.py for building the package. However, with the changes introduced by PEP517, we can now use more modern and cleaner tools, like Poetry.

This branch contains the current effort of moving Biotite from setuptools to Poetry. These are the issues that need to be resolved:

• [x] Filling the pyproject.toml with the necessary information
• [x] Adding a build script for compilation of extension modules
• [x] Resolve interaction with conda (python-poetry/poetry#105, python-poetry/poetry#190, python-poetry/poetry#1432)
• [ ] Stabilize build system for C-extensions (python-poetry/poetry#2740)
• [x] Document changes to the build system
• [x] Document changes pytest/sphinx invocation (Poetry does not use a setup.py)
• [x] Update CI for Travis CI
• [x] Update CI for AppVeyor