Fix TracebackFrameProxy.set_next() on Python 3.7

[vstinner]

Fix issue #1050: fix a crash in TracebackFrameProxy.set_next() on
Python 3.7 and newer, when Python is build in debug mode.

Since Python 3.7, traceback.tb_next field can be modified: ctypes is
no longer needed.

[vstinner]

Example on my Fedora 30 with python3-debug = Python 3.7.4.

Without this change, Python does crash:

$ python3-debug -m venv env
$ env/bin/python setup.py install
$ env/bin/python -m pip install pytest
$ env/bin/python -m pytest --tb=short -Werror 
...
$ env/bin/python -m pytest --tb=short -Werror -s tests/test_api.py 
======================================================================== test session starts ========================================================================
platform linux -- Python 3.7.4, pytest-5.1.0, py-1.8.0, pluggy-0.12.0
rootdir: /home/vstinner/jinja, inifile: setup.cfg
collected 27 items                                                                                                                                                  

tests/test_api.py ......../builddir/build/BUILD/Python-3.7.4/Modules/gcmodule.c:263: update_refs: Assertion "((gc)->gc.gc_refs >> (1)) != 0" failed.
object  : <refcnt 0 at 0x7fe081241660>
type    : traceback
refcount: 0
address : 0x7fe081241660
Fatal Python error: Aborted

Current thread 0x00007fe08fa41680 (most recent call first):
  File "/home/vstinner/jinja/jinja2/nodes.py", line 171 in iter_child_nodes
  File "/home/vstinner/jinja/jinja2/visitor.py", line 43 in generic_visit
  File "/home/vstinner/jinja/jinja2/visitor.py", line 39 in visit
  (...)

With this change, the whole test suite pass:

vstinner@apu$ env/bin/python -m pytest --tb=short -Werror 
=================================== test session starts ====================================
platform linux -- Python 3.7.4, pytest-5.1.0, py-1.8.0, pluggy-0.12.0
rootdir: /home/vstinner/jinja, inifile: setup.cfg, testpaths: tests
collected 582 items                                                                        

tests/test_api.py ...........................                                        [  4%]
tests/test_async.py ............................................                     [ 12%]
tests/test_asyncfilters.py ....................................                      [ 18%]
tests/test_bytecode_cache.py ...                                                     [ 18%]
tests/test_core_tags.py ............................................................ [ 29%]
..                                                                                   [ 29%]
tests/test_debug.py ....                                                             [ 30%]
tests/test_ext.py .........................................                          [ 37%]
tests/test_features.py .s                                                            [ 37%]
tests/test_filters.py .............................................................. [ 48%]
.......................................                                              [ 54%]
tests/test_idtracking.py .....                                                       [ 55%]
tests/test_imports.py ...........                                                    [ 57%]
tests/test_inheritance.py ................                                           [ 60%]
tests/test_lexnparse.py ............................................................ [ 70%]
.............................................                                        [ 78%]
tests/test_loader.py .................s..                                            [ 81%]
tests/test_nativetypes.py ..............                                             [ 84%]
tests/test_regression.py ................s........................                   [ 91%]
tests/test_security.py ...............                                               [ 93%]
tests/test_tests.py ...........................                                      [ 98%]
tests/test_utils.py ........                                                         [100%]

============================== 579 passed, 3 skipped in 3.13s ==============================

[vstinner]

Thanks ;-)

[davidism]

No problem, I also merged it up to master. This will go out when 2.10.2 is released, but I can't make a guarantee on when that will be.