Add pyproject.toml to allow building sdist, wheel

This allows anyone to `python -m build` an sdist or wheel for Hancho.
In particular, @aappleby should be able to use this config as-is if
they choose to publish Hancho on PyPI.

The original `hancho.py` remains standalone and independent, and can
still be copied into project development trees as-is. If @aappleby puts
hancho on PyPI, anyone will be able to install and update hancho via
`pip install` or whatever dependency management system they like.
It also helps system package maintainers to package Hancho for their
distribution with little modification.

As a PEP 517 build backend, I suggest poetry-core rather than setuptools
because poetry-core specifically supports packaging single-file `*.py`
modules that live in a project root directly, while the setuptools
backend would have required a subdirectory. This makes sure that
`hancho.py` remains a portable, self-contained script that doesn’t
require PyPI, pip, or anything else, and can be copied anywhere you
like.

One real-life example of a working system package using the Poetry
backend is the `hancho` AUR package. [1] [2] [3]

Addresses aappleby#1. [4]

[1]: https://aur.archlinux.org/packages/hancho

[2]: https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=hancho

[3]: https://aur.archlinux.org/cgit/aur.git/tree/pyproject.toml.template?h=hancho

[4]: aappleby#1

.gitignore

@@ -2,4 +2,5 @@ bin
 obj
 gen
 build
+dist
 __pycache__

pyproject.toml

@@ -0,0 +1,35 @@
+[build-system]
+requires = ["poetry-core>=1.9.0"]
+build-backend = "poetry.core.masonry.api"
+
+[tool.poetry]
+name = "hancho"
+version = "0.0.5"
+description = "A simple, pleasant build system"
+license = "MIT"
+authors = ["aappleby"]
+readme = ["README.md", "docs/README.md"]
+repository = "https://github.com/aappleby/hancho"
+classifiers = [
+    "Intended Audience :: Developers",
+    "Topic :: Software Development :: Build Tools",
+    "Topic :: Utilities",
+]
+include = [
+    { path = "docs", format = "sdist" },
+    { path = "tests", format = "sdist" },
+    { path = "tutorial", format = "sdist" },
+    { path = "examples", format = "sdist" },
+]
+exclude = ["**/.gitignore"]
+
+[tool.poetry.dependencies]
+python = "^3.10"
+
+[tool.poetry.scripts]
+hancho = "hancho:app.main"
+
+[tool.poetry.urls]
+"Additional documentation" = "https://github.com/aappleby/hancho/tree/main/docs#readme"
+"Tutorial" = "https://github.com/aappleby/hancho/tree/main/tutorial#readme"
+"Issue tracker" = "https://github.com/aappleby/hancho/issues"