This removes any doubts about what type of string is in use.

This allows the library to be built and tested with a non-default
version of CPython, e.g.

scons bindings=python python=python3.6
scons bindings=python python=python3.6 testpython

In Python 3.x

- int and long types are unified. The unified type is called int.
- the text string type (unicode) is renamed to str.
- the byte string type (str) is renamed to bytes.
- chr returns a text string (i.e. str)
- xrange is renamed to range.
- dict.has_key() is removed
-

These have no effect in Python 3.x, they are the default. Enabling them
in Python 2.x, enabling them in Python 2.x allows single source
compatiblity.

We wrap all remaining references to __attribute__ into a macro call to
H_GCC_ATTRIBUTE, to allow compilation with MSVC CL.EXE