Update Sphinx to 3.5.2, sphinx_rtd_theme to 0.5.1 and sphinx-tabs to 2.1.0 (#4572)

Co-authored-by: Hugo Locurcio <[email protected]>

requirements.txt

@@ -3,11 +3,11 @@
 # Sync with readthedocs:
 # https://github.com/readthedocs/readthedocs.org/blob/master/requirements/pip.txt
 # https://github.com/readthedocs/readthedocs.org/blob/master/requirements/local-docs-build.txt
-sphinx==1.8.5  # pyup: <2.0.0
-sphinx_rtd_theme==0.4.3
+sphinx==3.5.2
+sphinx_rtd_theme==0.5.1
 
 # Code tabs extension for GDScript/C#
-sphinx-tabs==1.1.13
+sphinx-tabs==2.1.0
 
 # Custom 404 error page (more useful than the default)
 sphinx-notfound-page==0.6