Add packaging via Poetry

[ffe4]

Not sure if you are still interested, but this PR would enable packaging via Poetry.

If you are not familiar with poetry, after installing it and merging this PR you could run the following to build your package:

$ poetry build
Building ebml (0.1.0)
  - Building sdist
  - Built ebml-0.1.0.tar.gz
  - Building wheel
  - Built ebml-0.1.0-py3-none-any.whl

Similarly, run poetry publish to publish the package on PyPI. You can pip install the result from https://test.pypi.org/project/ebml/ and run the tests to verify. The only changes I made to the repo were moving the module files to a subdirectory and adding the pyproject.toml.

If you want to publish the package it would be worth updating the README and some metadata for PyPI as well. Just wanted to check if you are even still here first.