Modernize setuptools support to use pyproject.toml

Replace the obsolete `setup.py` script and `setup.cfg` configuration
file with the modern PEP 621 `pyproject.toml` metadata.  This also
implies replacing the custom version/long-description grabbing logic
(that is broken with Python 3.12) with the features nowadays built-in
in setuptools.

Fixes Knio#172

pyproject.toml

@@ -0,0 +1,42 @@
+[build-system]
+requires = ["setuptools"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "dominate"
+authors = [
+    {name = "Tom Flanagan", email = "[email protected]"},
+    {name = "Jake Wharton"},
+]
+description = "Dominate is a Python library for creating and manipulating HTML documents using an elegant DOM API."
+readme = {file = "README.md", content-type = "text/markdown"}
+requires-python = ">=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*"
+keywords = ["framework", "templating", "template", "html", "xhtml", "python", "html5"]
+license = {text = "LGPL-3.0-or-newer"}
+classifiers = [
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: GNU Lesser General Public License v3 (LGPLv3)",
+    "Operating System :: OS Independent",
+    "Programming Language :: Python",
+    "Programming Language :: Python :: 2",
+    "Programming Language :: Python :: 2.7",
+    "Programming Language :: Python :: 3",
+    "Programming Language :: Python :: 3.4",
+    "Programming Language :: Python :: 3.5",
+    "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 :: Implementation :: PyPy",
+    "Topic :: Internet :: WWW/HTTP :: Dynamic Content",
+    "Topic :: Software Development :: Libraries :: Python Modules",
+    "Topic :: Text Processing :: Markup :: HTML",
+]
+dynamic = ["version"]
+
+[project.urls]
+Homepage = "https://github.com/Knio/dominate/"
+
+[tool.setuptools.dynamic]
+version = {attr = "dominate._version.__version__"}