Patch contributed by Corey Brune.

The exception is the import related questions at the end, which
need to be rewritten anyway.

ISO C90 forbids mixed declarations and code

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).