fix: add missing build-system

[branchvincent]

This causes missing metadata for source builds (since build-system will default to setuptools which won't pickup the metadata from poetry)

$ pip install --no-binary=:all: decli
Collecting decli
  Downloading decli-0.6.1.tar.gz (7.4 kB)
  Installing build dependencies ... done
  Getting requirements to build wheel ... done
  Installing backend dependencies ... done
  Preparing metadata (pyproject.toml) ... done
Discarding https://files.pythonhosted.org/packages/2e/9c/b76485e6120795c8b632707bafb4a9a4a2b75584ca5277e3e175c5d02225/decli-0.6.1.tar.gz (from https://pypi.org/simple/decli/) (requires-python:>=3.7): Requested decli from https://files.pythonhosted.org/packages/2e/9c/b76485e6120795c8b632707bafb4a9a4a2b75584ca5277e3e175c5d02225/decli-0.6.1.tar.gz has inconsistent version: expected '0.6.1', but metadata has '0.0.0'

[woile]

Thanks for the PR! Can you try removing 3.7 from the CI?
https://github.com/woile/decli/blob/master/.github/workflows/test.yaml#L15

[branchvincent]

@woile no prob, removed