Flake8 fixes
Fixed #10461 -- bug in generic relation + annotate() case

This issue was fixed when the contenttype restriction was moved from
where clause to the join clause. So, this is tests only addition.
Fixed #18419 -- Full backwards compatibility for old language codes

Improved documentation about zh-* deprecation and upgrade path.

Thanks to Baptiste Mispelon for the code reviews.
Fix python2 breakage caused by e5e044da87800feb6ef63fef1765d8c05022d926.
lru_cache is most performant with maxsize=None, otherwise it will include
locking for thread-safety.
Added memorizing test (fails with memoize)
Added backport author/license/link
Removed memoize example tests (added to ticket for future reference)
python3 fixes
Fix deprecation test