travis: we should hear it if Py3.6-dev ever starts failing. (And we should eventually remove it, but it feels too early right now.)

Hard switch off IPython test if import fails. Could not find a better way to keep it from failing on appveyor.

Split Python compatibility adaptation code out of ModulePreamble section and into its own separate section to allow moving it around and support using CYTHON_INLINE etc. in it.

PyPy3: Register module cleanup with atexit

Travis-CI: Setup macOS builds

Trivial typo fixes

Most are non-user facing. 
Found using:
`codespell -d -q 3`

Related to #1838.

Use complete IPython dependency path in test runner to work around current test failures under appveyor.

Closes #1930.

Simplify preprocessor code that avoids the "fallthrough" C compiler warnings, assuming that "__has_attribute()" is widely supported these days. At least GCC 5.x and clang seem to have it.
Closes #1930.

Closes #1930.

Fix the first argument special method signatures of "__eq__", "__lt__" and the other richcmp methods to be of "self" type, in accordance with to the CPython specs.
See https://docs.python.org/3/reference/datamodel.html#emulating-container-types
Closes #1935.

BF: closes 1484 for unordered map