sphinx is not needed to run tests.

--- watchdog-6.0.0/requirements-tests.txt.orig
+++ watchdog-6.0.0/requirements-tests.txt
@@ -1,4 +1,2 @@
 flaky==3.8.1
 pytest==8.3.3
-sphinx==7.4.7; python_version <= "3.9"
-sphinx==8.1.3; python_version > "3.9"