Remarshal

The YAML 1.2 upgrade in version 0.18 caught some downstream users off guard when their output documents were no longer valid YAML 1.1. We could have signaled big breaking changes with an increment to the major version, but Remarshal was still version 0.x. We should be done with minor breaking changes now. Let's not postpone version 1.0 because our new YAML library ruamel.yaml isn't stable. We will release version 1.0 soon. Going forward, we will be able to use semantic versioning to signal breaking changes.

Breaking changes

- **Python API:**
The API has undergone significant changes in how formatting options are handled.
- The formatting parameters to `remarshal.remarshal` have been removed and replaced with format-specific option classes. It makes clear what format uses what options.
- While this is a breaking change, it should have no impact as Remarshal is used through its command-line interface.
- `--json-indent` and `--yaml-indent` are now synonyms. See below for why. This should also have minimal impact.

Non-breaking changes

- **New Python output format:**
Added support for outputting data as Python code.
- **Unified formatting options:**
There are several improvements to make the formatting options more consistent.
- A new unified `--indent` option. The option acts as a synonym for `--json-indent` and `--yaml-indent`. The old options remain available, though they have been hidden in the help message.
- A new `--width` option acts as a synonym for `--yaml-width`. Besides YAML, it sets the width for Python output.
- Improved the limitations section in the readme.

- YAML: Emit `null` instead of an empty scalar for nulls. In 0.18.0 Remarshal switched to an empty scalar (meaning, `foo:` with no value after the key) to represent nulls. This was the result of migrating to the ruamel.yaml library, where it is the default. While both representations are [valid YAML 1.2](https://yaml.org/spec/1.2-old/spec.html#id2786563), people tend to write and expect `null`. The current release restores pre-0.18.0 behavior.

- YAML: Fixed error handling that broke in version 0.18.0. Improved the error messages.

- BREAKING CHANGE: Dropped support for Python 3.8 and PyPy 3.9.
- TOML: Remarshal has stopped emitting empty tables when they have subtables.

Breaking changes:

- Remarshal 0.18 finally encodes and decodes YAML as **YAML 1.2** rather than YAML 1.1 like earlier versions. It means no more "Norway problem" (unquoted `NO`, `No`, and `no` being decoded as boolean false). Remarshal can now correctly process GitHub Actions YAML. To implement this, we have migrated from PyYAML to [ruamel.yaml](https://pypi.org/project/ruamel.yaml/). The option `--sort-keys` no longer affects YAML output due to a lack of support in the new YAML library. Because ruamel.yaml is not yet considered stable, the 1.0 release of Remarshal is postponed.
- Multiple small changes to the Python API when using Remarshal as a library. For example, the order of the arguments to the function `remarshal` has changed. A type checker like mypy or Pyright can help you upgrade. We hope to make this the biggest change before the API is declared stable in version 1.0.
- Support for Python 3.7 has been dropped.

Non-breaking changes:

- **TOML decoding speed-up:** TOML decoding should be at least 10× faster on average. Encoding is unaffected.
- There are new format command-line options `-f`/`--from` and `-t`/`--to` like in Pandoc and iconv(1). We have hidden (but kept) `-if` and `-of` with a single dash. Those are legacy options from the Go days of Remarshal.
- A new `--verbose` command-line option has been added for debugging.
- The command-line interface and the help message have color. Color is used sparingly. You can disable it by setting the environment variable `NO_COLOR`.
- Remarshal is now tested with PyPy and on macOS.