Change sphinx pin to != 1.8.0

The callable function typing issue should be fixed in sphinx commit
bbb4593bfaa65611b6bd600c5, so the next release after 1.8.0 should
contain the fix.

Fixed in GH sphinx-doc/sphinx#5431

https://github.com/sphinx-doc/sphinx/commit/bbb4593bfaa65611b6bd600c5e37c4c54af87f1a
1 file changed