Convert unicode index to long, not int, in get_char_index

There's an error in the `PyFT2Font.get_char_index()` method
added in 2d56ffeb . The type for the unicode index to be sent
to `FT_Get_Char_Index` is `FT_ULong` - an unsigned long - but
the `PyArg_ParseTuple` call that converts it from Python used
`I` in the format string, which converts a Python int to a C
unsigned int, not a C unsigned long. This doesn't seem to cause
a problem on little-endian arches, but it results in completely
incorrect conversion on big-endian arches, which in turn would
result in wrong glyphs, unfound glyphs, and even in an infinite
recursion in `UnicodeFonts._get_glyph`.

To get correct conversion we must use `k` not `I`, which is
the specifier for a C unsigned long.

Ref: https://docs.python.org/3/c-api/arg.html#numbers

Author: AdamWill

src/ft2font_wrapper.cpp

@@ -971,7 +971,7 @@ static PyObject *PyFT2Font_get_char_index(PyFT2Font *self, PyObject *args, PyObj
     FT_UInt index;
     FT_ULong ccode;
 
-    if (!PyArg_ParseTuple(args, "I:get_char_index", &ccode)) {
+    if (!PyArg_ParseTuple(args, "k:get_char_index", &ccode)) {
         return NULL;
     }