[Distutils] A possible refactor/streamlining of PEP 517

[Thomas Kluyver]

Thank-you all for the discussion and the attempts to accommodate flit,
but I'll bow out now. It's become clear that the way flit approaches
packaging is fundamentally incompatible with the priorities other people
have for the ecosystem. Namely, I see sdists as archival artifacts to be
made approximately once per release, but the general trend is to make
them a key part of the build pipeline.

Making a guerilla tool with no concern for integration was fun. It
became frustrating as people began to use it and expected it to play
well with other tools, so I jumped on PEP 517 as a way to bring it into
the fold. That didn't work out, and a tool that doesn't play well with
pip can only be an attractive nuisance at best, even if it technically
complies with the relevant specs.

Flit is therefore deprecated, and I recommend anyone using it migrate
back to setup.py packaging.

Best wishes,
Thomas