[web/test] Modernize unittest_propertysheet.py - remove call to os.makedirs in global scope - fold CACHEDIR as an instance attribute - bonus: use unittest directly instead of lgc.testlib

[web/webconfig] Drop pre-3.9 backward compatibility method "has_resource"

[web] Stop using negative indices with os.stat results The python doc says: More items may be added at the end by some implementations. .st_mtime is also more readable than [-2].

[doc] small rst fixes and cleanups in changelog files Related to #4832808