use sphinx <3 (workaround for m2r)

Author: espdev

setup.py

@@ -33,7 +33,7 @@ def get_long_description():
         'Click>=7.0,<8',
     ],
     extras_require={
-        'docs': ['sphinx>=2.3', 'Pallets-Sphinx-Themes', 'm2r'],
+        'docs': ['sphinx>=2.3, <3', 'Pallets-Sphinx-Themes', 'm2r'],
         'tests': ['pytest'],
     },
     url='https://github.com/click-contrib/click-option-group',