Fix build-system in pyproject.toml

[mgorny]

Fix the build-system.requires key to specify poetry-core rather than poetry as the correct package providing poetry.core.* backend. Remove setuptools and wheel requirements since they are not used when building via the PEP517 backend.

[pitbulk]

As you can see in the CI, if I don't add it on build-system.requires, there is an strange issue where the error

ImportError: No module named 'setuptools'

is raised, even if the setuptools was installed in the very beginning with the command

Run pip install -U setuptools

See pypa/setuptools#2980

[mgorny]

The problem is that you're running Python < 3.6. While poetry might support it still (I didn't check), pip does not, so it's installing some old version (i.e. the last version supporting Python 2.7/3.5) and that version is simply broken with PEP517 builds.

[pitbulk]

@mgorny Soon I plan to deprecate support on python 2.7, 3.4 and 3.5 on this toolkit (only security patch could be supported) so gonna be a good time to apply this PR and improve the code properly with new features available in python > 3.5