Replace setup.py and requirements.txt with pyproject.toml. Add pylgl as
a dependency and simplify the SAT solver interface: drop incremental API
(add_clause, assume, solve iterator) in favor of a single solve(cnf)
method. Add CNF.dimacs() to expose clauses in DIMACS format. Add
Makefile for build, install, test, and lint targets. Replace README.rst
with README.md.