build-system in pyproject.toml: build-backend must be a string

Hi, [we are having a problem with recent python 3.11](https://aur.archlinux.org/packages/picopt#comment-913719) in this line when we install using [build frontend](https://github.com/pypa/build/blob/0.10.0/src/build/__init__.py#L221) (the same with `pip install picopt` method). https://github.com/ajslater/picopt/blob/11acd82f8e1af137066225346cd324703d9aea83/pyproject.toml#L3 I understand that [python 3.11 only accepts a string, not a list](https://realpython.com/python311-tomllib/#learn-basic-toml).