pyproject.toml was missing.

pyproject.toml

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+[project]
+name = "regex"
+version = "2023.5.2"
+authors = [
+    {name="Matthew Barnett", email="regex@mrabarnett.plus.com"},
+]
+description = "Alternative regular expression module, to replace re."
+readme = "README.rst"
+requires-python = ">=3.6"
+classifiers = [
+    "Development Status :: 5 - Production/Stable",
+    "Intended Audience :: Developers",
+    "License :: OSI Approved :: Apache Software License",
+    "Operating System :: OS Independent",
+    "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",
+    "Topic :: Scientific/Engineering :: Information Analysis",
+    "Topic :: Software Development :: Libraries :: Python Modules",
+    "Topic :: Text Processing",
+    "Topic :: Text Processing :: General",
+]
+license = {file = "LICENSE.txt"}
+
+[build-system]
+requires = ["setuptools>=61.0"]
+build-backend = "setuptools.build_meta"
+
+[project.urls]
+"Homepage" = "https://github.com/mrabarnett/mrab-regex"
+"Bug Tracker" = "https://github.com/mrabarnett/mrab-regex/issues"