Pyproject-metadata

- Make license and readme files `pathlib.Path` instances
- Add the license contents to the metadata file
- Add support for multiline data in metadata fields

- Renamed project to `pyproject_metadata`
- Support multiple clauses in requires-python
- Error out when dynamic fields are defined
- Update dynamic field when setting version

- Use Core Metadata 2.1 if possible
- Fix bug preventing empty README and license files from being used

- Avoid core metadata `Author`/`Maintainer` fields in favor of
`Author-Email`/`Maintainer-Email`

- Fix Python version requirement

- Add documentation