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.

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

Include changelog in Sphinx documentation

Add python_requires to setup.py

See #1874.

Update Python version classifiers

Avoid useless None check when testing non-None containers for emptiness, e.g. for the result of a comprehension.