Revert Python version requirement to 3.8+

Zensical is a dev-only dependency for building documentation and should
not constrain the runtime Python version. The library itself supports
Python 3.8+.

Changes:
- Revert requires-python from >=3.10 to >=3.8
- Add Python version marker to zensical dependency (>= 3.10)
- Restore Python 3.8 and 3.9 classifiers
- Update documentation to reflect Python 3.8+ support

ReadTheDocs build continues to use Python 3.10 (specified in
.readthedocs.yml), which satisfies Zensical's requirement for
documentation builds.

Author: claude

docs/index.md

@@ -31,7 +31,7 @@ the features I was missing.
 
 Required dependencies:
 
-* Python 3.10 or later
+* Python 3.8 or later
 * xarray 2024 or later
 
 Install from PyPI in your virtual environment:

pyproject.toml

@@ -5,10 +5,12 @@ description = "A lightweight xarray validation library"
 authors = [{ name = "Vincent Leroy", email = "vincent.leroy@rayference.eu" }]
 dependencies = ["attrs", "numpy", "xarray"]
 readme = "README.md"
-requires-python = ">= 3.10"
+requires-python = ">= 3.8"
 classifiers = [
   "Development Status :: 4 - Beta",
   "License :: OSI Approved :: MIT License",
+  "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",
@@ -32,7 +34,7 @@ units = ["pint"]
 [dependency-groups]
 lint = ["ruff>=0.14.0"]
 docs = [
-  "zensical>=0.0.21",
+  "zensical>=0.0.21; python_version >= '3.10'",
   "mkdocstrings-python>=1.0",
 ]
 test = ["pytest-cov", "pytest", "xdoctest"]