Use flit-core<3.9,>=3.4 as the build backend

[BeautyyuYanli]

Use flit-core<3.9,>=3.4 as the build backend, so that PEP517 and PEP621 can be available on Python >= 3.6 That means:

• pip install . is available in Python 3.6
• The output wheel of pdm build is available in Python 3.6
• pyproject.toml is available on Python 3.6

[wey-gu]

For now(with httpx dependency, to support centOS7 py3.6 build from source), this is the best way, but let's hold this change(not merge) until we have some time to verify the full nebula-ent build(need to not remove pyproject.toml in makefile)