This commit contains the implementation of PEP570: Python positional-only parameters.

* Update Grammar/Grammar with new typedarglist and varargslist

* Regenerate grammar files

* Update and regenerate AST related files

* Update code object

* Update marshal.c

* Update compiler and symtable

* Regenerate importlib files

* Update callable objects

* Implement positional-only args logic in ceval.c

* Regenerate frozen data

* Update standard library to account for positional-only args

* Add test file for positional-only args

* Update other test files to account for positional-only args

* Add News entry

* Update inspect module and related tests

This reverts commit b36e5d62.

* Add _PyEval_FiniThreads2(). _PyEval_FiniThreads() now only clears
  the pending lock, whereas _PyEval_FiniThreads2() destroys the GIL.
* pymain_free() now calls _PyEval_FiniThreads2().
* Py_FinalizeEx() now calls _PyEval_FiniThreads().

PyEval_AcquireLock() and PyEval_AcquireThread() now
terminate the current thread if called while the interpreter is
finalizing, making them consistent with PyEval_RestoreThread(),
Py_END_ALLOW_THREADS, and PyGILState_Ensure().

https://bugs.python.org/issue36745

* Fix typo in test_cyclic_gc subtest
* Improve test coverage for xml.etree.ElementTree

In the process of converting the date.fromtimestamp function to use
argument clinic in GH-8535, the C API for PyDate_FromTimestamp was
inadvertently changed to expect a timestamp object rather than an
argument tuple.

This PR fixes this backwards-incompatible change by adding a new wrapper
function for the C API function that unwraps the argument tuple and
passes it to the underlying function.

This PR also adds tests for both PyDate_FromTimestamp and
PyDateTime_FromTimestamp to prevent any further regressions.

The new markup is currently ignored by IDLE's tk doc display.

Py_TRACE_REFS ABI is incompatible with release and debug (Py_DEBUG)
ABI.

Misc/python-config.sh lives in the build directory, not in the source
directory.

Add PYTHONPATH=$(srcdir) to run $(PYTHON_FOR_REGEN) -m Parser.pgen,
so it's possible to build Python from a different directory.

Add _Py_INIT_HAS_EXITCODE() macro.

Regrtest.finalize() now closes explicitly the WindowsLoadTracker
instance.

Properly handle a test which fail but then pass.

Add test_rerun_success() unit test.

regrtest now always detects uncollectable objects. Previously, the
check was only enabled by --findleaks. The check now also works with
-jN/--multiprocess N.

--findleaks becomes a deprecated alias to --fail-env-changed.

Rewrite run_tests_multiprocess() function as a new MultiprocessRunner
class with multiple methods to better report errors and stop
immediately when needed.

Changes:

* Worker processes are now killed immediately if tests are
  interrupted or if a test does crash (CHILD_ERROR): worker
  processes are killed.
* Rewrite how errors in a worker thread are reported to
  the main thread. No longer ignore BaseException or parsing errors
  silently.
* Remove 'finished' variable: use worker.is_alive() instead
* Always compute omitted tests. Add Regrtest.get_executed() method.

Py_FinalizeEx() now clears _PyRuntime.warnings variables and
_PyRuntime.exitfuncs.

Changes:

* Add _PyWarnings_Fini(): called by Py_FinalizeEx()
* call_ll_exitfuncs() now clears _PyRuntime.exitfuncs while iterating
  on it (on backward order).

_PyThreadState_Delete() has no return value.

* Add TestResult and MultiprocessResult types to ensure that results
  always have the same fields.
* runtest() now handles KeyboardInterrupt
* accumulate_result() and format_test_result() now takes a TestResult
* cleanup_test_droppings() is now called by runtest() and mark the
  test as ENV_CHANGED if the test leaks support.TESTFN file.
* runtest() now includes code "around" the test in the test timing
* Add print_warning() in test.libregrtest.utils to standardize how
  libregrtest logs warnings to ease parsing the test output.
* support.unload() is now called with abstest rather than test_name
* Rename 'test' variable/parameter to 'test_name'
* dash_R(): remove unused the_module parameter
* Remove unused imports

Add 'state' or 'runtime' parameter to functions in gcmodule.c to
avoid to rely directly on the global variable _PyRuntime.

Add 'gilstate', 'runtime' or 'xidregistry' parameter to many
functions on pystate.c to avoid lying on _PyRuntime global.

In debug build, import now also looks for C extensions compiled in
release mode and for C extensions compiled in the stable ABI.

On Unix, C extensions are no longer linked to libpython.

It is now possible to load a C extension built using a shared library
Python with a statically linked Python.

When Python is embedded, libpython must not be loaded with
RTLD_LOCAL, but RTLD_GLOBAL instead. Previously, using RTLD_LOCAL, it
was already not possible to load C extensions which were not linked
to libpython, like C extensions of the standard library built by the
"*shared*" section of Modules/Setup.

distutils, python-config and python-config.py have been modified.

Document that subprocess.Popen no longer raise an exception on error
like missing program on very specific platforms when using
os.posix_spawn() is used.

bpo-28552, bpo-7774: Fix distutils.sysconfig if sys.executable is
None or an empty string: use os.getcwd() to initialize project_base.

Fix also the distutils build command: don't use sys.executable if
it's evaluated as false (None or empty string).

Fix Py_DEBUG constant: check for sys.gettotalrefcount attribute
rather than sys.getobjects.

Update also SpecialBuilds.txt documentation.