Seeing during PR composition how basically every mode check and every `test.support.temp_umask` use is broken by ACLs, I'm starting to doubt that fixing individual test cases is the way to go.

Though we can simply not worry about supporting everything imaginable and solve problems as they come and declare highly unusual cases unsupported.

For the record, other potential problems:

* All open(O_CREAT) and umask uses anywhere in the tests are broken by default.
    * Though only 4 test files were affected for now -- test_pathlib, test_tarfile, test_os, test_import .
* There are other overlay security frameworks that override POSIX permissions like NFSv4 and SELinux.

And possible solutions:

* Skip mode_t checks outright if overlay security is detected like it's already done in Windows
* Embed cleanup code into test.support's temp_* and such. In the POSIX.1e case, will have to create temporary dirs or change permissions for the current dir.
    * I don't think regrtest's temporary dir is a good place for this 'cuz tests are supposed to be runnable directly with `unittest`, too.