Added tests to ensure that all defined opcodes are supported.

Patch by Neeraj Badlani.

External importers were being added in both phases of the import
system initialisation.

They're only supposed to be added in the second phase, after the
import machinery has been appropriately configured.

* bpo-33251: ConfigParser.items no longer returns items present in vars.

Documentation for `ConfigParser.items()` states:
'Items present in vars no longer appear in the result.'

This fix aligns behaviour to that specified in the documentation.

Starting with glibc 2.27.9000-xxx, sigaddset() can return EINVAL for some
reserved signal numbers between 1 and NSIG.  The `range(1, NSIG)` idiom
is commonly used to select all signals for blocking with `pthread_sigmask`.
So we ignore the sigaddset() return value until we expose sigfillset()
to provide a better idiom.

This change contains minor things that make diffing between Lib/tokenize.py and
Lib/lib2to3/pgen2/tokenize.py cleaner.

Also update PIL doc references to Pillow.

Upgrade ensurepip to bundle pip 10.0.1

Setup modules are no longer built with -DPy_BUILD_CORE by default,
as using that flag may now require including additional internal-only header files.

Instead, only the modules that specifically need it use that setting.

Fix clang ubsan (undefined behavior sanitizer) warnings in dictobject.c by
adjusting how the internal struct _dictkeysobject shared keys structure is
declared.

This remains ABI compatible.  We get rid of the union at the end of the
struct being used for conveinence to avoid typecasting in favor of char[]
variable length array at the end of a struct. This is known to clang to be
used for variable sized objects and will not cause an undefined behavior
problem.  Similarly, char arrays do not have strict aliasing undefined
behavior when cast.

PEP-007 does not currently list variable length arrays (VLAs) as allowed
in our subset of C99.  If this turns out to be a problem, the fix to this is
to change the char `dk_indices[]` into `dk_indices[1]` and restore the
three size computation subtractions this change removes:
  `- Py_MEMBER_SIZE(PyDictKeysObject, dk_indices)`

If this works as is I'll make a separate PR to update PEP-007.

as docstrings and translatable strings, and rejects
bytes literals and f-string expressions.

Converting with line_info=False and col_info=True crashed before.

`test_urllib2net.OtherNetworkTests.test_sites_no_connection_close`
used `http://www.imdb.com/` but it is moved to https so the test is
not valid anymore.  Skip test for the moment to allow CI to proceed.

In fact, we now require a newer Sphinx version because APIs have moved around.

This note in documentation was never true.

* Now uses pickle protocol 4

* Doesn't wrap the grammar's `__dict__` in ordered dictionaries anymore as
dictionaries in Python 3.6+ are ordered by default

This still produces deterministic pickles (that hash the same with MD5).
Tested with different PYTHONHASHSEED values.