Add pyproject.toml declaring setuptools as build backend

[jgerityneurala]

Description

This changeset adds a minimal pyproject.toml to the project, bringing it into line with PEP 517 and making it harder to build the project in an environment with incompatible versions of setuptools and setuptools-scm, as these packages will be installed together as part of the build-requires in the isolated build environment.

Fixes #110

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• After adding this file, the project can be successfully built/installed by the same virtual environment and pip install invocation as listed in seqeval build is broken on setuptools<61 (Python 3.8) #110.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules