Fix psycopg2 (un)instrument_connection to use weakref, not mutate connection object (#4257)

* Fix psycopg2 (un)instrument_connection to use weakref, not mutate object

* Changelog

* conditional import for docs types

* Lint

* SImplify test

* Simplify

* Fix docs

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Co-authored-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>