devel/py-archspec: Fix BUILD_DEPENDS

from pyproject.toml:
[build-system]
requires = ["poetry-core>=1.0.0"]
build-backend = "poetry.core.masonry.api"

Approved by:	portmgr (blanket)
With hat:	python
Reference:	https://github.com/archspec/archspec/blob/v0.2.1/pyproject.toml#L42-L44