Remarshal

* remarshal now works with the latest verson of TOML, 1.0.0-rc1.
* Python 2.7 is no longer supported. Python 2.7-specific code has been removed.
* Fixes have been made to YAML error handling.

This is a technical release that doesn't impact users. After a long period of using legacy tooling, remarshal has migrated to [Poetry](https://python-poetry.org/).

* Support for the [CBOR](https://cbor.io/) format has been implemented.
* The output file argument is correctly named.

* Support for the [MessagePack](https://msgpack.org/) format has been implemented.
* The flag `--preserve-key-order` now has a short form: `-p`.
* The internals have been refactored to make added new formats easier.
* More conversion errors are handled.

This release improves the usability of the command-line interface.

* Remarshal now tries to determine the input and the output format by the file extension.
* The output file can be given as a positional argument. For example, you can use `remarshal input.toml output.json` or `toml2json input.toml output.json` instead of `remarshal input.toml -o output.json` or `toml2json input.toml --output output.json`.
* `--if` and `--of` have been added as synonyms for `-if`/`--input-format` and `-of`/`--output-format` respectively. Two dashes with a two-letter flag are more standard. `-if` and `-of` are deprecated, but will not be removed at least until v1.0.0.
* **BREAKING CHANGE:** `--indent-json` now requires a number. Replace existing uses of `--indent-json` with `--indent-json 2` for identical behavior.
* The shebang in `remarshal.py` has been changed to `! /usr/bin/env python3` from `! /usr/bin/env python`. Full Python 2.7 compatibility remains.

* Require PyYAML 5.1 or later.