Fixed #21840 -- Moved dunder methods from SimpleLazyObject to LazyObject.

This commit also added tests for LazyObject and refactored
the testsuite of SimpleLazyObject so that it can share
test cases with LazyObject.
Simplified implementation of collectstatic command.

Since d2e242d16c6dde6f4736086fb38057424bed3edb made isinstance()
calls work correctly on LazyObject, we can simplify the
implementation of is_local_storage added in
7e27885c6e7588471fd94a4def16b7081577bdfc.
Merge pull request #2221 from bmispelon/LazyObject-refactor

Fixed #21840 -- Moved dunder methods from SimpleLazyObject to LazyObject...