Switch kgb to pyproject.toml.

This modernizes kgb's packaging, moving from `setup.py` to
`pyproject.toml`. This maintains Python 2/3 compatibility, but allows us
to use modern `pip` to put this into an editable install and stay
compliant with modern Python practices.

The metadata has changed to now explicitly list Python 3.12 and 3.13 as
supported versions.

Testing Done:
Successfully built source dists and wheels, verifying the package
contents and the Python 2/3 compatibility metadata.

Reviewed at https://reviews.reviewboard.org/r/14172/

pyproject.toml

@@ -0,0 +1,64 @@
+[build-system]
+requires = ['setuptools']
+build-backend = 'setuptools.build_meta'
+
+[project]
+name = 'kgb'
+description = 'Utilities for spying on function calls in unit tests.'
+authors = [
+    {name = 'Beanbag, Inc.', email = '[email protected]'},
+]
+license = { text = 'MIT' }
+readme = 'README.rst'
+requires-python = '>=2.7,!=3.0.*,!=3.1.*,!=3.2.*,!=3.3.*,!=3.4.*,!=3.5.*'
+dynamic =  ['version']
+
+keywords = [
+    'pytest',
+    'unit tests',
+    'spies',
+]
+
+classifiers = [
+    'Development Status :: 5 - Production/Stable',
+    'Environment :: Other Environment',
+    'Framework :: Pytest',
+    'Intended Audience :: Developers',
+    'License :: OSI Approved :: MIT License',
+    'Operating System :: OS Independent',
+    'Programming Language :: Python',
+    'Programming Language :: Python :: 2',
+    'Programming Language :: Python :: 2.7',
+    'Programming Language :: Python :: 3',
+    'Programming Language :: Python :: 3.6',
+    'Programming Language :: Python :: 3.7',
+    'Programming Language :: Python :: 3.8',
+    'Programming Language :: Python :: 3.9',
+    'Programming Language :: Python :: 3.10',
+    'Programming Language :: Python :: 3.11',
+    'Programming Language :: Python :: 3.12',
+    'Programming Language :: Python :: 3.13',
+    'Topic :: Software Development',
+    'Topic :: Software Development :: Libraries :: Python Modules',
+    'Topic :: Software Development :: Testing',
+]
+
+
+[project.urls]
+Homepage = 'https://github.com/beanbaginc/kgb'
+Documentation = 'https://github.com/beanbaginc/kgb'
+Repository = 'https://github.com/beanbaginc/kgb'
+
+
+[project.entry-points."pytest11"]
+kgb = 'kgb.pytest_plugin'
+
+
+[tool.setuptools.dynamic]
+version = { attr = 'kgb.get_package_version' }
+
+
+[tool.setuptools.packages.find]
+where = ['.']
+include = ['kgb*']
+namespaces = false