While the directives in the source files are still called `distutils`, the [Python upstream documentation](https://docs.python.org/3/library/distutils.html) does not
recommend using distutils anymore, and rather points at setuptools, so avoid confusing new users by providing confusing legacy usage examples.

Also, this corrects one example in which Extension is imported too late, and would cause annoying errors when running `setup.py`.

The idea being that struct-types like memoryviews
can generate their own reference counting code
using a common interface with Python objects.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/iterator.html).

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/iterator.html).

- CyFunction seems to lead to deeper stacks on recursion.
- Passing integers through call layers can end up creating new int objects instead of keeping the identical objects ('is' test fails)'.

Closes #2514.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/gen.html).

Closes #2514.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/gen.html).

PySlice_Unpack() and PySlice_AdjustIndices() were missing from
slice.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/slice.html).

PySlice_Unpack() and PySlice_AdjustIndices() were missing from
slice.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/slice.html).

Closes GH-3418.

Original change https://github.com/cython/cython/pull/1927

Exclude failing "complex.h" test on Windows, where old MSVC versions lack "<complex.h>" entirely and more recent versions fail to compile the test.

Reasoning being that this make it easier to catch pypy3
regressions as they happen.

* Fixed some very simple pypy3 failures (largely to do with
  different exception strings)
* Splits pypy3_bugs.txt into three files
  - one for bugs that cause hard crashes (which we don't want to
    run in Travis at all);
  - one for bugs that are probably unfixable because they're just
    due to implementation details (e.g. when destructors are
    called).
  - all other bugs remain in pypy3_bugs.txt
  (The categorization has been done fairly quickly, so some bugs
  may be in the wrong place)
* Made sure (hopefully) all bugs are now categorized, so a basic
  runtests.py with pypy3 should hopefully pass
* Changed pypy3 to be required in Travis
* Added an extra (optional) test that runs through pypy3_bugs.txt.
  The majority of this is expected to fail. This requires an
  extra option to runtest.py "--listfile", which just runs through
  the tests listed in the file.

I haven't made pypy2 a required test in this commit - since Python2 support is deprecated soon, there seemed limited value in putting much effort into pypy2.

Added faulthandler to runtests in the hope of being able to pin-down segmentation faults better on Travis

FileListExcluder matches regexes, not just name. Uses the same mechanism as is used for processing string passed on commandline