Revert to sphinx-tabs 3.4.0

Revert sphinx-tabs back to 3.4.0 to fix dependency conflict. 
The conflict is caused by:
- sphinx 4.4.0 depends on docutils<0.18 and >=0.14
- sphinx-rtd-theme 1.1.1 depends on docutils<0.18
- sphinx-tabs 3.4.1 depends on docutils~=0.18.0

requirements.txt

@@ -7,7 +7,7 @@ sphinx==4.4.0
 sphinx_rtd_theme==1.1.1
 
 # Code tabs extension for GDScript/C#
-sphinx-tabs==3.4.1
+sphinx-tabs==3.4.0
 
 # Custom 404 error page (more useful than the default)
 sphinx-notfound-page==0.8.3