Don't require sphinx spelling to be installed, for readthedocs benefit

docs/conf.py

@@ -19,6 +19,11 @@ try:
 except ImportError:
     sphinx_rtd_theme = None
 
+try:
+    from sphinxcontrib import spelling
+except ImportError:
+    spelling = None
+
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the
@@ -38,9 +43,11 @@ extensions = [
     'sphinx.ext.intersphinx',
     'sphinx.ext.viewcode',
     'cryptography-docs',
-    'sphinxcontrib.spelling',
 ]
 
+if spelling is not None:
+    extensions.append('sphinxcontrib.spelling')
+
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']