importlib.abc.Loader.exec_module() is also defined.

Before this change, create_module() was optional **and** could return
None to trigger default semantics. This change now reduces the
options for choosing default semantics to one and in the most
backporting-friendly way (define create_module() to return None).

atomic memory access if available. Patch written by Vitor de Lima and Gustavo
Temple.

* Document why set_result() calls are safe
* Cleanup gather(). Use public methods instead of hacks to consume the
  exception of a future.
* sock_connect(): pass directly the fd to _sock_connect_done instead of the
  socket.

Patch by A.M. Kuchling.

Patch by A.M. Kuchling.

Patch by Martin Panter to clean up some problems with the merge
of the codecs docs changes from Python 3.4.

- clarified the distinction between text encodings and other codecs
- clarified relationship with builtin open and the io module
- consolidated documentation of error handlers into one section
- clarified type constraints of some behaviours
- added tests for some of the new statements in the docs

loader.loadTestsFromModule() logged errors.

availability of the function is checked during the compilation. Patch written
by Bernard Spil.