I've added:
#define PycairoContext_GET(obj)    (((PycairoContext *)(obj))->ctx)

Added an extra "()" round obj, changed pycairo_context to one word, and
used some uppercase - I've been following the PEP 7 "Style Guide for C code",
where it suggests:

    - Public functions and variables use MixedCase with underscores,
      like this: PyObject_GetAttr, Py_BuildValue, PyExc_TypeError.

    - Macros should have a MixedCase prefix and then use upper case,
      for example: PyString_AS_STRING, Py_PRINT_RAW.

Cool :-)