docs: sphinx-rtd-theme as optional dep, specify needs_sphinx

- Set sphinx-rtd-theme as optional docs dependency
- Specify needs_sphinx and needs_extensions in the conf.py configuration file (see https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-needs_sphinx)

docs/source/conf.py

@@ -32,7 +32,8 @@
 # -- General configuration
 
 # If your documentation needs a minimal Sphinx version, state it here.
-# needs_sphinx = '1.3'
+needs_sphinx = "6.0"
+needs_extensions = {"sphinx_rtd_theme": "1.0"}
 
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom ones.

pyproject.toml

@@ -36,7 +36,7 @@ all = ["ema_workbench[jupyter,dev,cov,docs,graph,parallel]"]
 jupyter = ["jupyter", "jupyter_client", "ipython", "ipykernel"]
 dev = ["pytest", "pytest-mock", "jupyter_client", "ipyparallel"]
 cov = ["pytest-cov", "coverage", "coveralls"]
-docs = ["sphinx", "nbsphinx", "myst", "pyscaffold", "myst-parser"]
+docs = ["sphinx", "sphinx-rtd-theme", "nbsphinx", "myst", "pyscaffold", "myst-parser"]
 graph = ["altair", "pydot", "graphviz"]
 parallel = ["ipyparallel", "traitlets"]