pyproject.toml: pyminifier as build-backend

[nschloe]

Modern Python projects define their build requirements in pyproject.toml, e.g.,

[build-system]
requires = [
  "setuptools>=42",
  "wheel",
]
build-backend = "setuptools.build_meta"

It'd be nice addition of pyminifier could be defined as the builder, e.g.,

[build-system]
requires = [
  "setuptools>=42",
  "wheel",
  "pyminifier>=xyz",
]
build-backend = "pyminifier.build_meta"

to spit out a minified package code.