chore: migrate from setup.py to pyproject.toml

Modernize build system to use pyproject.toml as the single source of
truth for package metadata and version. Replace custom build scripts
with pk3 CLI commands.

Changes:
- Add `pyproject.toml` with static version and metadata
- Remove `setup.py` and `_building/build_setup.py`
- Remove `_building/install.sh` and `_building/publish.sh`
- Update `_building/common.mk` to use `pk3 tag` and `pk3 publish`
- Use `importlib.metadata` in `__init__.py` for runtime version

Author: drmingdrmer

__init__.py

@@ -11,7 +11,9 @@
 
 """
 
-__version__ = "0.1.14"
+from importlib.metadata import version
+
+__version__ = version("k3git")
 __name__ = "k3git"
 
 from .gitopt import GitOpt

_building/build_setup.py

@@ -23,11 +23,11 @@ static_check:
 readme:
 	python _building/build_readme.py
 
-build_setup_py:
-	PYTHONPATH="$$(cd ..; pwd)" python _building/build_setup.py
+release:
+	pk3 tag
 
 publish:
-	./_building/publish.sh
+	pk3 publish
 
 install:
-	./_building/install.sh
+	pip install -e .

_building/common.mk

@@ -0,0 +1,56 @@
+[build-system]
+requires = ["setuptools>=61.0"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "k3git"
+version = "0.1.14"
+description = "wrapper of git command-line"
+readme = "README.md"
+license = {text = "MIT"}
+requires-python = ">=3.6"
+authors = [
+    { name = "Zhang Yanpo", email = "drdr.xp@gmail.com" }
+]
+keywords = ["git", "cli", "commandline"]
+classifiers = [
+    "Development Status :: 4 - Beta",
+    "Intended Audience :: Developers",
+    "Topic :: Software Development :: Libraries",
+    "License :: OSI Approved :: MIT License",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.6",
+    "Programming Language :: Python :: 3.7",
+    "Programming Language :: Python :: 3.8",
+    "Programming Language :: Python :: Implementation :: PyPy",
+]
+dependencies = [
+    "k3str>=0.1.1,<0.2",
+    "k3ut>=0.1.7,<0.2",
+    "k3handy>=0.1.7,<0.2",
+    "k3fs>=0.1.5,<0.2",
+]
+
+[project.urls]
+Homepage = "https://github.com/pykit3/k3git"
+Documentation = "https://k3git.readthedocs.io"
+
+[project.optional-dependencies]
+dev = [
+    "pytest>=7.0",
+    "ruff",
+]
+publish = [
+    "build",
+    "twine",
+    "pk3",
+]
+
+[tool.setuptools]
+packages = ["k3git"]
+
+[tool.setuptools.package-dir]
+k3git = "."
+
+[tool.ruff]
+line-length = 120