Add support for the limited C API (Py_LIMITED_API) (#184)

Examples of API not available in the limited C API:

* PyBytesWriter
* PyConfig_Get(), PyConfig_GetInt().
* PyDict_ContainsString().
* PyFrame_GetBack(), PyFrame_GetVar()
* PyLongWriter_Create() and PyLong_Export()
* PyObject_VisitManagedDict(), PyObject_ClearManagedDict()
* PyThreadState_GetUnchecked()
* PyTime_t API.
* PyUnicodeWriter
* PyUnstable_SetImmortal().
* Py_SETREF(), Py_XSETREF().

Implementation notes:

* PyUnicode_EqualToUTF8AndSize() has no ASCII fast path.
* PyLong_GetSign(), PyLong_IsPositive(), PyLong_IsNegative() and
  PyLong_IsZero() are implemented with PyObject_RichCompareBool().
* PyBytes_Join() calls PyObject_CallMethod(sep, "join", iterable).
* Add PYTHONCAPI_COMPAT_EXPECT_TYPE() macro to raise TypeError.
  There are 3 implementations depending on the API and Python
  version.

test_pythoncapi_compat.py now reuses the setup module to list tests.

Closes #183

Author: vstinner