* defined(__STDC_NO_THREADS__) can be trusted only when using glibc > 2.12

* Add __GLIBC__ defined check.

- Replace npy_3kcompat macro PyInt_AsLong by appropriate definition
- Replace npy_3kcompat macro PyInt_AS_LONG by appropriate definition.
- Reformat code for readability.

The code in extensively reformated in this PR which may hide the most
important parts of the diff, but I needed to do it to make the code
readable.

MAINT: Replace remaining PyString macros.

Replace the npy_3kcompat macro PyNumber_Int by its definition.

Replace the npy_3kcompat macro PyInt_FromLong by its definition.

Replace remaining npy_3kcompat PyString macros with their definition.

- PyString_AS_STRING -> PyBytes_AS_STRING
- PyString_GET_SIZE -> PyBytes_GET_SIZE

This replaces the npy_3kcompat macro PyStringObject with its definition.

Replaces the npy_3kcompat PyString_Check macro with its definition.

We no longer need to use the compatibility function after dropping
support for Python 2.7. In some cases unicode was the correct string
type rather than the bytes of the compatibility version and bugs in the
array `__complex__` and array `__array_interface__` methods have been
fixed by changing that.