fix: declare sphinx-rtd-theme in docs extras (#253)

Currently sphinx-rtd-theme is incorrectly being included in the main
group dependencies and hence pulls in itself and sphinx as a runtime dep
for the client. Declaring it in the docs group ensures it is only
installed if the `sdcclient[docs]` is installed.

Signed-off-by: Dominic Evans <[email protected]>

pyproject.toml

@@ -28,7 +28,7 @@ flake8 = "^3.8.4"
 coverage = "^6.2"
 
 [tool.poetry.extras]
-docs = ["sphinx", "sphinx-material"]
+docs = ["sphinx", "sphinx-material", "sphinx-rtd-theme"]
 
 [tool.poetry-dynamic-versioning]
 enable = true