prevent global timezone state from leaking out of test cases.
Merge pull request #12633 from senny/no_more_time_zone_leaks

prevent global timezone state from leaking out of test cases.
only warn on leaked time zone state instead of rasing an error.

Raising `RuntimeErrors` skips important cleanup code and leads to
a lot of subsequent errors. This clutters the test output with a lot
of noise.
prevent `time_zone_aware_attributes` test leak. follow-up to #12633.
cast json values on write to be consistent with reading from the db.

See also commit 5ac2341fab689344991b2a4817bd2bc8b3edac9d
Merge pull request #12643 from severin/pg_cast_json_on_write

cast json values on write to be consistent with reading from the db.