When a file ends with a line that contains a line continuation character
the text of the emitted SyntaxError is empty, contrary to the old
parser, where the error text contained the text of the last line.

Add _Py_EnsureTstateNotNULL(tstate) macro: call Py_FatalError() if
tstate is NULL, the error message contains the current function name.

This updates _PyErr_ChainStackItem() to use _PyErr_SetObject()
instead of _PyErr_ChainExceptions(). This prevents a hang in
certain circumstances because _PyErr_SetObject() performs checks
to prevent cycles in the exception context chain while
_PyErr_ChainExceptions() doesn't.

When an asyncio.Task is cancelled, the exception traceback now
starts with where the task was first interrupted.  Previously,
the traceback only had "depth one."

_PyErr_ChainExceptions() now ensures that the first parameter is an
exception type, as done by _PyErr_SetObject().

* The following function now check PyExceptionInstance_Check() in an
  assertion using a new _PyBaseExceptionObject_cast() helper
  function:

  * PyException_GetTraceback(), PyException_SetTraceback()
  * PyException_GetCause(), PyException_SetCause()
  * PyException_GetContext(), PyException_SetContext()

* PyExceptionClass_Name() now checks PyExceptionClass_Check() with an
  assertion.

* Remove XXX comment and add gi_exc_state variable to _gen_throw().

* Remove comment from test_generators

The PyThreadState_GetFrame() function now returns a strong reference
to the frame.

Add the dependency to tstate more explicit.

Co-authored-by: Victor Stinner <vstinner@python.org>