* Trivial cleanups following bpo-31370

* Also cleanup the "importlib._bootstrap_external" module

Skip the file if it has bad encoding.

* 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()

Issue 27948: Allow backslashes in the literal string portion of f-strings, but not in the expressions. Also, require expressions to begin and end with literal curly braces.

Development is moving to https://github.com/ilevkivskyi/com2ann

Patch by Ivan Levkivskyi.

Issue 27921: Remove backslash from another f-string. I'll revert this change before beta 2. I also need to look in to why test_tools/test_unparse fails with the files that are now being skipped.

Closes issue 27921: Disallow backslashes anywhere in f-strings. This is a temporary restriction. In 3.6 beta 2, the plan is to again allow backslashes in the string parts of f-strings, but disallow them in the expression parts.

Patch by Jon Dufresne.

Patch by Guo Ci Teo.

Temporary hack for issue #25180: exclude test_fstring.py from the unparse round-tripping, while I figure out how to properly fix it.

Patch by Christie Wilson.

Patch by Michael McFadden, with a few small style tweaks.

Adds `load_package_tests` function to test.support, uses it in test_asyncio,
test_email, test_json, test_tools, test_importlib and all test_importlib
sub-packages to implement test discovery.