Update sphinxcontrib-spelling from 4.2.0 to 4.2.1

Author: pyup-bot

dev-requirements.txt

@@ -21,7 +21,7 @@ pytest==4.3.1  # Test runners
 PyYAML==5.1
 Sphinx==1.8.5
 sphinx-autodoc-typehints==1.6.0
-sphinxcontrib-spelling==4.2.0
+sphinxcontrib-spelling==4.2.1
 twine==1.13.0
 versioneer==0.18
 vulture==1.0