Switch to flit

After #393, our two remaining required dependencies will both be using flit as their build backend. Switching to flit would eliminate two build dependencies (setuptools and wheel) and will simplify bootstrapping build without setuptools. Flit is also able to generate a setup.py from its metadata which we can version for setuptools holdouts.

[pradyunsg]

With installer getting a CLI (https://installer.readthedocs.io/en/latest/cli/installer/) and flit_core getting better bootstrapping story pretty soon (https://github.com/pypa/flit/pull/511), I think this is now unblocked for moving to flit.

I think migrating to flit would also bring a fairly clean boostrapping story as well.

• Build and install flit_core, through flit_core’s boostrappers.
• Build installer with flit_core, and install with flit_core’s bootstrapper.
• Build build with flit_core, and install with installer.
• You have build and installer installed, and ready for use.
• python -m build . && python -m installer ./dist/*.whl will install the project in cwd into the running Python version.

[vishwin]

flit_core is absolutely self-bootstrapping now. installer installs itself after flit_core generates the wheel in installer’s working copy.

if packaging switches to Flit again as I think was planned, then they can all be built with flit_core and installed with installer,

I’ve nudged at pypa/packaging#340.