Revert to setup

[vincentsarago]

closes #471

pyproject.toml + flit broke the namespace package (replacing titiler.core by titiler_core)

[vincentsarago]

Note: I found a way to make it work using setuptools

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

[tool.setuptools.dynamic]
version = {attr = "titiler.core.__version__"}

[tool.setuptools]
include-package-data = true

But right now setuptool support of pyproject.toml is marked as experimental. we might re-try pyproject + build system later!