fix(deps): downgrade Sphinx to 8.1.3 for Python 3.10 compatibility

Resolve ReadTheDocs build failure caused by Sphinx version incompatibility.

**Root Cause**:
- .readthedocs.yaml specifies Python 3.10 (aligns with pyproject.toml minimum)
- requirements.txt had Sphinx==8.2.3 (requires Python >=3.11)
- Incompatibility: Sphinx 8.2.3 cannot install on Python 3.10

**Changes**:
- requirements.txt: Sphinx 8.2.3 → 8.1.3 (Python 3.10 compatible)
- requirements-dev.txt: Add version constraint `sphinx>=7.3,<8.2`
  (prevents future freezes from re-introducing incompatibility)

**Validation**:
- Local docs build: SUCCESS (7 pre-existing warnings, no errors)
- Sphinx 8.1.3 compatible with Python 3.10+
- docstring-inheritance extension: Loads correctly

**Context**:
Previous commit (0a0ce18) correctly added docstring-inheritance to docs
and aligned .readthedocs.yaml with Python 3.10 minimum, but exposed
latent Sphinx version incompatibility from automated requirements freeze.

This fix ensures:
- RTD builds succeed with Python 3.10 + Sphinx 8.1.3
- Future freezes stay within compatible range
- Documentation requirements align with package minimum Python version

Refs: https://readthedocs.org/projects/solarwindpy/builds/

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: blalterman

requirements-dev.txt

@@ -19,7 +19,7 @@ doc8
 flake8-docstrings
 pydocstyle
 numpydoc
-sphinx
+sphinx>=7.3,<8.2
 sphinx_rtd_theme
 sphinxcontrib-spelling
 sphinxcontrib-bibtex

requirements.txt

@@ -4,7 +4,7 @@
 
 Bottleneck==1.6.0
 PyYAML==6.0.2
-Sphinx==8.2.3
+Sphinx==8.1.3
 astropy==7.1.0
 black==25.1.0
 doc8==2.0.0