Refactor package layout to modern Python packaging

- Convert setup.py/setup.cfg to pyproject.toml
- Implement PEP 420 implicit namespaces (remove namespace declarations)
- Delete namespace-only __init__.py files
- Add /build to .gitignore

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: rnixx

.gitignore

@@ -7,3 +7,4 @@
 /dist/
 /htmlcov/
 /requirements-mxdev.txt
+/build

pyproject.toml

@@ -0,0 +1,53 @@
+[build-system]
+requires = ["setuptools>=61.0"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "node"
+version = "1.2.3.dev0"
+description = "Building data structures as node trees"
+readme = "README.rst"
+license = {text = "Simplified BSD"}
+authors = [{name = "Node Contributors", email = "dev@conestack.org"}]
+keywords = ["node", "tree", "fullmapping", "dict"]
+classifiers = [
+    "Development Status :: 5 - Production/Stable",
+    "License :: OSI Approved :: BSD License",
+    "Intended Audience :: Developers",
+    "Operating System :: OS Independent",
+    "Topic :: Software Development",
+    "Programming Language :: Python",
+    "Programming Language :: Python :: 2.7",
+    "Programming Language :: Python :: 3.7",
+    "Programming Language :: Python :: 3.8",
+    "Programming Language :: Python :: 3.9",
+    "Programming Language :: Python :: 3.10",
+]
+dependencies = [
+    "odict>=1.9.0",
+    "plumber>=1.5",
+    "zope.component",
+    "zope.deferredimport",
+    "zope.deprecation",
+    "zope.lifecycleevent",
+]
+
+[project.optional-dependencies]
+test = [
+    "pytest",
+]
+
+[project.urls]
+Homepage = "http://github.com/conestack/node"
+
+[tool.setuptools.packages.find]
+where = ["src"]
+
+[tool.setuptools]
+zip-safe = true
+
+[tool.setuptools.package-dir]
+"" = "src"
+
+[tool.zest-releaser]
+create-wheel = true