bpo-29972: Skip tests known to fail on AIX (#979)

* bpo-29972: Fix test_eintr on AIX

On AIX, sigtimedwait(0.2) sleeps 199.8 ms, whereas the test expects
200 ms or longer.

* bpo-29972: Skip some inet_pton() tests on AIX

Skip some inet_pton() tests of test_socket on AIX.

inet_pton() on AIX is less strict than on Linux and doesn't reject
some invalid IP addresses. The unit tests test more the libc than
Python itself.

* bpo-29972: Skip tests known to fail on AIX

* test_locale.test_strcoll_with_diacritic()
* test_locale.test_strxfrm_with_diacritic()
* test_strptime.test_week_of_year_and_day_of_week_calculation()
* test_tools.test_POT_Creation_Date()