Update pyproject.toml so it validates.

[hexmode]

While working with the Poetry's proposed support for the project section in pyproject.toml, it identified the following problems:

$ poetry check

The Poetry configuration is invalid:
  - project.license must be valid exactly by one definition (0 matches found)
  - Either [project.version] or [tool.poetry.version] is required in package mode.

Since the license field is awkward, is not required, and is specified in the classifiers, it could be removed.

The second issue from poetry check is probably a non-issue since the version is dynamic.