gh-131031: Fix separated running of pickle tests

[donBarbos]

I found that tests stopped running successfully directly after this commit (PR #122373)

• Issue: test_pickle tests failing when invoked directly #131031

[donBarbos]

cc @tomasr8 (sorry for choosing you but current pickle expert is inactive)

[tomasr8]

Are you sure the assertions are being executed? Since self.dumps(obj, proto) raises, anything that comes after in the with block will not execute.

[donBarbos]

@tomasr8 sorry for the first commit.
As far as I know, changing __main__ is bad practice, but it seems that this issue cannot be solved otherwise, since the problem is that __main__ is different for different launch methods, and I was inspired by a similar PR #131371

[tomasr8]

This looks ok, but I'm not expert in his area :/ @serhiy-storchaka could you have a look?

[serhiy-storchaka]

Good PR, but using a global class which is not imported in test_pickle.py instead of AbstractPickleTests would fix the tests too. See #133356.

Faking the __main__ module looks heavyweight, we don't know what side effects can it have. We would use this approach if there was no other way.

[serhiy-storchaka]

You can use support.swap_item(). It is already used in other tests to fake a module.