install only the minimum required to build our docs for rtd (#4133)

* install only the minimum required to build our docs for rtd

We don't need to install sphinxcontrib-spelling or doc8 in rtd.

* move another package
diff --git a/setup.py b/setup.py
index 800e94d..b655816 100644
--- a/setup.py
+++ b/setup.py
@@ -296,12 +296,14 @@
         ":platform_python_implementation != 'PyPy'": ["cffi >= 1.7"],
 
         "test": test_requirements,
+        "docs": [
+            "sphinx >= 1.6.5",
+            "sphinx_rtd_theme",
+        ],
         "docstest": [
             "doc8",
             "pyenchant >= 1.6.11",
             "readme_renderer >= 16.0",
-            "sphinx >= 1.6.5",
-            "sphinx_rtd_theme",
             "sphinxcontrib-spelling >= 4.0.1",
         ],
         "pep8test": [