native pyproject.toml config

[gaborbernat]

This is a ticket to track and implement a native pyproject.toml configuration. Will update with the proposed plan and format.

Core goals:

• explicitly inheritance based (a lot of substitution at the moment is used to replicate inheritance),
• substitution able,
• reasonably familiar (e.g. share most concepts with ini format),
• succinct (some form of factors needs to have).

We'll use this opportunity to rename some keys (e.g. skipsdist should be skip_packaging, etc).

[obestwalter]

Learning from the old format, I would add:

• easy to debug (good visualization of the settings - `--showconfig`` 2.0)
• conservative error handling (err on the side of crashing early with a helpful error, rather than being too allowing for "creative" uses)

[obestwalter]

... and most importantly: a clear, versioned specification that can then evolve as we for sure won't get it right at first try :)