faulthandler._read_null() is not reliable: it does not crash on AIX.

sigwaitinfo() can be interrupted on Linux (raises InterruptedError), but not on
AIX.

when \r\n appears at end of 65535 bytes without other newlines.

* Use mixins for generating tests for different compression types.
* Make test_tarfile discoverable.
* Use more special tests (i.e. assertEqual, assertIs) instead of assertTrue.
* Add explicit test skips instead of reporting skipped tests as passed.
* Wrap long lines.
* Correct a comment for test_hardlink_extraction1.
* Add support.requires_gzip.

and some other minor enhancements.

Also, improve some docstring descriptions of the 'errors' parameter.

Closes #14015.

exec runs into an error.  Prevent a regular multi-close of the /dev/null
fd when any of stdin, stdout and stderr was set to DEVNULL.

Reported by Atsuo Ishimoto.

Original patch by Sarah K.

AI_NUMERICSERV isn't defined on OSX 10.4.

encoding now always contains an encoding name.

changing IOError to OSError (#16715).

Fix bug in existing human test and add instructions; fix two bugs in tested
code; remove redundancies, add spaces, and change two internal method names.
Add mock_tk with mocks for tkinter.Variable subclasses and tkinter.messagebox.
Use mocks in test_config_name to unittest methods that are otherwise gui-free.

socket.gethostname() now decodes the hostname from the locale encoding, instead
of using the UTF-8 encoding in strict mode.

os.path.join(*self.dirs) produces an invalid path on windows.
ftp paths are always forward-slash seperated like this. /pub/dir.

by using the wording from the Py2.7 docs.