Versioningit

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- Added custom setuptools commands for inserting the project version into a
source file at build time
- New step and subtable: "onbuild"
- New public `get_cmdclasses()` and `run_onbuild()` functions
- Moved documentation from the README to a Read the Docs site
- Established external documentation for the public library API
- When falling back to using `tool.versioningit.default-version`, emit a
warning if the version is not PEP 440-compliant.
- The `versioningit` command now honors the `VERSIONINGIT_LOG_LEVEL`
environment variable

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- Changes to custom methods:
- The signatures of the method functions have changed; user-supplied
parameters are now passed as a single `params: Dict[str, Any]` argument
instead of as keyword arguments.
- User-supplied parameters with the same names as step-specific method
arguments are no longer discarded.
- Changes to the "git-archive" method:
- Lightweight tags are now ignored (by default, but see below) when
installing from a repository in order to match the behavior of the
`%(describe)` format placeholder.
- The "match" and "exclude" settings are now parsed from the
`describe-subst` parameter, which is now required, and the old `match`
and `exclude` parameters are now ignored.
- Git 2.35's "tags" option for honoring lightweight tags is now recognized.
- Added a dedicated error message when an invalid `%(describe)` placeholder
is "expanded" into itself in an archive
- The `file` parameter to the "basic" write method is now required when the
`[tool.versioningit.write]` table is present. If you don't want to write the
version to a file, omit the table entirely.
- Library API:
- `Config` is no longer exported; it should now be considered private.
- Merged `Versioningit.from_config()` functionality into
`Versioningit.from_project_dir()`
- Renamed `Versioningit.from_config_obj()` to `Versioningit.from_config()`;
it should now be considered private

-------------------
- Git 1.8.0 is now the minimum required version for the git methods, and this
is documented. (Previously, the undocumented minimum version was Git 1.8.5.)
- Document the minimum supported Mercurial version as 5.2.

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- Call ``importlib.metadata.entry_points()`` only once and reuse the result for
a speedup (contributed by [jenshnielsen](https://github.com/jenshnielsen))

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- Support Python 3.10
- Support tomli 2.0

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- Gave the CLI interface an `-n`/`--next-version` option for showing a
project's next release version
- Added a `get_next_version()` function
- Added a mention to the README of the existence of exported functionality
other than `get_version()`
- Renamed the individual step-calling methods of `Versioningit` to have names
of the form `do_$STEP()`