migrate to pyproject.toml

[branchv]

The project currently fails under PEP 517 build isolation since setup.py imports from pypager which imports prompt_toolkit:

$ pip install --no-binary=:all: pypager
# ...
ModuleNotFoundError: No module named 'prompt_toolkit'
# ...

As a fix, this migrates to pyproject.toml (the modern replacement for setup.py favoring static metadata).