textproc/py-pyphen: Fix BUILD_DEPENDS

from pyproject.toml:
[build-system]
requires = ['flit_core >=3.2,<4']
build-backend = 'flit_core.buildapi'

Approved by: portmgr (blanket)
With hat: python
Reference: https://github.com/Kozea/Pyphen/blob/0.14.0/pyproject.toml#L1-L3