textproc/py-sphinx: Switch to use flit-core as build system

In Sphinx-5.3.0/pyproject.toml build system is specified as following.

[build-system]
requires = ["flit_core>=3.7"]
build-backend = "flit_core.buildapi"

I confirmed build succeeds with devel/py-flit-core as BUILD_DEPENDS.
And it also reduces the number of packages that are required as build
dependencies.

PR: 273577
Approved by: sunpoet (python, maintainer)