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

* Work around a C compiler warning (in PyPy's cpyext).
* Avoid a C compiler warning in PyPy3.
* Fix a C compiler warning about an unused variable when compiling in PyPy.

Closes #2643.

PyNumber_MatrixMultiply() and PyNumber_InPlaceMatrixMultiply() were
missing from number.pxd.

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

PyNumber_MatrixMultiply() and PyNumber_InPlaceMatrixMultiply() were
missing from number.pxd.

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

Fixes "Make it a bit less likely that the Python version of cython.cast() does something unexpected/wrong."

Strings werren't static for the limited API so had to be set at module init rather than compile time.

* Fixed issue where fused functions didn't register as cyfunctions

Issue https://github.com/cython/cython/issues/3384

This happened where Cython was using a shared API because the
CyFunction type imported from the shared api wasn't the same as
used to initialize the fused function tp_base.

* Fixed issue where fused functions didn't register as cyfunctions

Issue https://github.com/cython/cython/issues/3384

This happened where Cython was using a shared API because the
CyFunction type imported from the shared api wasn't the same as
used to initialize the fused function tp_base.

Default arguments are now copied when the bound fused function is created.

Default arguments are now copied when the bound fused function is created.

1. DefNode.has_fused_arguments was set too early (before
locals/annotations) were evalutated, so function was not treated
as fused.
2. When re-evaluating the specializations of the fused function
it was treated as a redefinition because the locals/annotation was
reapplied over the specialized type.
3. Including annotation as string (required changes to
StringNode.analyse_as_type), and extra tests for fused type defined
as cython.fused_type in the Py file

Closes #3331.

Closes #1427 

Closes #3331.

* Cleanup of string initialization code for limited API

Now appears in a few large blocks rather than

```
 # if CYTHON_LIMITED_API
   individual string line
 # endif
```

* Fixed issue with some unicode strings, e.g.

```
# cython: language_level=3str, binding=True

def non_ascii_str():
    s = 'ø\x20\u0020'
    assert isinstance(s, str)
    return s
```

"@subsitute: tempita" tag ensures that they are loaded in tempita utility code class

Use correct "Py_mp_subscript" type slot for getitem() method instead of "Py_mp_ass_subscript" (which is for assignments).