Introduce pyproject.toml + poetry

[stankudrow]

Addresses the issue #345

Checklist:

• Add tests that demonstrate the correct behaviour of the change. Tests should fail without the change. - no need to
• Add or update relevant docs, in the docs folder and in code. - no need to
• Ensure PR doesn't contain untouched code reformatting: spaces, etc.
• Run flake8 and fix issues.
• Run pytest no tests failed. See https://clickhouse-sqlalchemy.readthedocs.io/en/latest/development.html.

References:

• https://shillelagh.readthedocs.io/en/latest/development.html#creating-a-custom-sqlalchemy-dialect

[DaniilAnichin]

Highlighted required changes on entry-points; and we either need to put versions back in __init__.py, or to change the documentation to avoid error on the import.
The rest is more or less the question of taste

[DaniilAnichin]

The rest looks ok, I guess

[stankudrow]

Would like to favour the PR #348 instead of this one.