[1.6.x] Fixed #21487 -- Session cannot store tzinfo instances anymore.

Thanks filipp for the report.
[1.6.x] Fixed sentence structure in docs/internals/contributing/triaging-tickets.txt

Backport of 4a54b69e88 from master
[1.6.x] Fixed #21443 -- Cannot show debug info on PY3's importlib

Thanks productions@zaziork.co.uk for the review.
Backport of 18185724e6 from master.
[1.6.x] Noted that localmem is the default cache.

Backport of c456ea4ec8 from master
[1.6.x] Improved docs for ModelFormSet.clean().

Backport of 033b26173b from master