MNT: Add requires-python metadata

* Add requires-python metadata through the addition of setuptools's
  python_requires in setup.py.
   - c.f. https://peps.python.org/pep-0621/#requires-python
   - Set minimum required python to 3.8 as Python 3.7 is EOL as of 2023-06-27.
     c.f. https://devguide.python.org/versions/#unsupported-versions

* The addition of requires-python is to provide guards to keep older
  CPython versions from installing releases that could contain
  unrunnable code.

Author: matthewfeickert

setup.py

@@ -15,6 +15,7 @@
     long_description_content_type="text/markdown",
     url="https://data-apis.org/array-api-compat/",
     license="MIT",
+    python_requires=">=3.8",
     extras_require={
         "numpy": "numpy",
         "cupy": "cupy",