switch to using defaults for versioningit

[bmaranville]

The settings that are currently used for versioningit in the pyproject.toml specify that the "next" version should be used when doing a wheel build or local install.

This causes the unexpected behavior that if a developer makes a change to a repository file and then does a local install to test, it automatically versions the install with the next patch version, but this conflicts with the way we are incrementing our versions (local build will get e.g. version 1.0.1 before an actual release 1.0.1 is made with a new tag, which is how versioningit is supposed to increment the version.

The default behavior of versioningit is to create local versions based on the most recent tag, which is the correct thing to do here, I think. This PR removes our manual configuration of versioningit (which uses "next_version").

[mdoucet]

The default formats use {base_version} rather than {next_version}, so that may be the main change that was needed.