closes #19737
ref: #19737 (comment)

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It's not the prettiest code, but since it will be removed once PEP 800
gets accepted (or rejected), I figured it would be best to keep it
simple, so that we can easily revert it.

It is time to announce this (as still experimental obviously).

(cherry picked from commit d33c147)

This PR adds 2 new weakref primitives for weakref.proxy (1 and 2 arg)

(cherry picked from commit 8f2371a)

Incorrect processing in detect_undefined_bitmap could cause a ValueError
exception in emit_undefined_attr_check.

(cherry picked from commit fd05265)

(cherry picked from commit 9ae3e9a)

Did various edits, added a few additional sections, and reordered some
sections.

(cherry picked from commit 73fa69e)

Remove Unreleased from section title

(cherry picked from commit f0863a5)

Fixes #19836

Fix is trivial, I am surprised none of the existing tests caught this in
the original PR. After this is merged, I am going to add the test to
`master`.

…__ (#19733)

Fixes #19732

This is a simple check to point users in the right direction when they
get errors because their class uses `__new__` but they wrote stubs for
`__init__`. I don't feel strongly about the exact wording used here. I
also considered "Maybe you meant to define `__new__` instead of
`__init__`?".

(cherry picked from commit 530bdc5)

if a variable is Final but the value is not yet known at compile-time,
and that variable is used as an input to an f-string, the f-string will
incorrectly contain "None"

Fixes [mypyc#1140](mypyc/mypyc#1140)

(cherry picked from commit dce8e1c)

…Any` (#14708)

Changelog update for 1.18.2

Also updated the changelog to reflect the initial release being 1.18.1,
since we had to bump the version due to wheels failing.

This adds the cherry picked PRs mentioned in
#19764 (comment)

(cherry picked from commit 4301be1)