Extend list of PyPI classifiers in pyproject.toml (#480)

Extend the list of classifiers, mostly restore the ones that were lost
during the move from `setup.cfg` to `pyproject.toml`.

pyproject.toml

@@ -12,9 +12,20 @@ readme = "README.md"
 license = {text = "BSD-3-Clause"}
 keywords = ["geoscience", "geophysics"]
 classifiers = [
-    "Programming Language :: Python :: 3",
+    "Development Status :: 5 - Production/Stable",
+    "Intended Audience :: Science/Research",
+    "Intended Audience :: Developers",
+    "Intended Audience :: Education",
     "License :: OSI Approved :: BSD License",
+    "Natural Language :: English",
     "Operating System :: OS Independent",
+    "Topic :: Scientific/Engineering",
+    "Topic :: Software Development :: Libraries",
+    "Programming Language :: Python :: 3 :: Only",
+    "Programming Language :: Python :: 3.8",
+    "Programming Language :: Python :: 3.9",
+    "Programming Language :: Python :: 3.10",
+    "Programming Language :: Python :: 3.11",
 ]
 requires-python = ">=3.8"
 dependencies = [