patches by Matt Joiner and Alexey Kachayev.

- missing DECREF in PyModule_FromDefAndSpec2
- missing DECREF in PyType_FromSpecAndBases2
- missing DECREF in _testmultiphase module

Patch by Petr Viktorin

Known limitations of the current implementation:

- documentation changes are incomplete
- there's a reference leak I haven't tracked down yet

The leak is most visible by running:

  ./python -m test -R3:3 test_importlib

However, you can also see it by running:

  ./python -X showrefcount

Importing the array or _testmultiphase modules, and
then deleting them from both sys.modules and the local
namespace shows significant increases in the total
number of active references each cycle. By contrast,
with _testcapi (which continues to use single-phase
initialisation) the global refcounts stabilise after
a couple of cycles.

parameters and returns bytes in such situations (matching the os module APIs).

Issue #23955: Add pyvenv.cfg option to suppress registry/environment lookup for generating sys.path.
Also cleans up and secures getpathp.c

Also, deprecate formatargspec, formatargvalues, and getargvalues
functions.  Since we are deprecating 'getfullargspec' function in
3.5 (documentation only, no DeprecationWarning), it makes sense
to also deprecate functions designed to be directly used with it.

In 3.6 we will remove 'getargsspec' function (was deprecated since
Python 3.0), and start raising DeprecationWarnings in other
'getarg*' family of functions.  We can remove them in 3.7 or later.

Also, it is worth noting, that Signature API does not provide 100%
of functionality that deprecated APIs have.  It is important to do
a soft deprecation of outdated APIs in 3.5 to gather users feedback,
and improve Signature object.