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.

There's no reason a cookie should _ever_ contain pickled data. That's just asking for a critical security vulnerability. Back in Python2 there were helpers for doing that, but they're no more in Python3. Now coded_value is used when the value needs to be encoded for any reason.

Adds some working and markup fixes that I missed
in the initial commit for this issue.

(Follow-up to GH-6419)

The pydoc CLI assumed -m pydoc would add the empty string
to sys.path, and hence got confused when it switched to
adding the full initial working directory instead.

This refactors the pydoc CLI path manipulation to be
more testable, and ensures it won't accidentally
remove the standard library directory containing
pydoc itself from sys.path.

`MethodType` has the exact semantics that `ExitStack` needs,
so we can avoid creating a Python level closure.

The examples in argparse documentation use `error: too few arguments`
error message which was removed in this commit f97c59aa in 2011.

Executors in concurrent.futures accepted tasks after executor was shutdown by interpreter exit. Tasks were left in PENDING state forever. This fix changes submit to instead raise a RuntimeError.