test_multibytecodec_support. Improve also error message of the
test_errorhandle() of TestBase.

descriptor of a pipe closed in the parent process is valid in the child process
according to fstat(), but the mode of the file descriptor is invalid, and read
or write raise an error.

Add also requires_mac_ver() decorator to test.support.

constructor has failed, e.g. because of an undeclared keyword argument. Patch
written by Oleg Oshmyan.

iso2022_jp, iso2022_jp_2 and iso2022_kr

the following case: sys.stdin.read() stopped with CTRL+d (end of file),
raw_input() interrupted by CTRL+c.

The tests now pass all combinations of -O/-OO and -B.  See also #7071
and #6292 for previous variations on the same theme.

test_versionpredicate needs a skip when sys.flags.optimize is true, but
I don’t know how to make that work with a DocTestSuite.

_posixsubprocess doesn't exist rather than simply stubbing it out
after the fact.  This adds coverage for the RuntimeWarning as well as
using the pure python _create_pipe instead of using
_posixsubprocess.cloexec_pipe unintentionally with the pure python
code.

Ironically: I don't think any platform should ever actually _use_ the
pure Python subprocess code on POSIX platforms anymore.  This at least
tests it properly in this stable branch.  The pure python code for
this is likely to be removed in 3.3.