Require Sphinx >= 8.0

Read the Docs wants to install Sphinx 4.something.  This causes
problems with Python 3.13 because the `imghdr` module was removed
from the standard library around Sphinx 6.2 and the epub builder
relies on it.

Sphinx is at version 8 now.  We can safely ask for it.

Author: atwilso

documentation_build_environment.yml

@@ -26,7 +26,7 @@ dependencies:
   - jupyter
 
   # Documentation Dependencies
-  - sphinx
+  - sphinx>=8.0
   - sphinx_rtd_theme
   - nbsphinx>=0.7.1
   - pandoc>=2.5