Fix invalid function signatures for PySequenceMethods callbacks

Function casts hid casting between potentially incompatible type
signatures (using int instead of Py_ssize_t). As it happens this seems
not to have caused any bugs on supported platforms so this change is
mainly for correctness and to avoid problems in the future.

source/blender/python/bmesh/bmesh_py_types.c

@@ -3118,7 +3118,7 @@ static Py_ssize_t bpy_bmelemseq_length(BPy_BMElemSeq *self)
   }
 }
 
-static PyObject *bpy_bmelemseq_subscript_int(BPy_BMElemSeq *self, int keynum)
+static PyObject *bpy_bmelemseq_subscript_int(BPy_BMElemSeq *self, Py_ssize_t keynum)
 {
   BPY_BM_CHECK_OBJ(self);
 

source/blender/python/bmesh/bmesh_py_types_customdata.c

@@ -740,7 +740,8 @@ static PyObject *bpy_bmlayercollection_subscript_str(BPy_BMLayerCollection *self
   return NULL;
 }
 
-static PyObject *bpy_bmlayercollection_subscript_int(BPy_BMLayerCollection *self, int keynum)
+static PyObject *bpy_bmlayercollection_subscript_int(BPy_BMLayerCollection *self,
+                                                     Py_ssize_t keynum)
 {
   Py_ssize_t len;
   BPY_BM_CHECK_OBJ(self);

source/blender/python/bmesh/bmesh_py_types_meshdata.c

@@ -391,7 +391,7 @@ typedef struct BPy_BMDeformVert {
 /* Mapping Protocols
  * ================= */
 
-static int bpy_bmdeformvert_len(BPy_BMDeformVert *self)
+static Py_ssize_t bpy_bmdeformvert_len(BPy_BMDeformVert *self)
 {
   return self->data->totweight;
 }

source/blender/python/bmesh/bmesh_py_types_select.c

@@ -163,7 +163,7 @@ static Py_ssize_t bpy_bmeditselseq_length(BPy_BMEditSelSeq *self)
   return BLI_listbase_count(&self->bm->selected);
 }
 
-static PyObject *bpy_bmeditselseq_subscript_int(BPy_BMEditSelSeq *self, int keynum)
+static PyObject *bpy_bmeditselseq_subscript_int(BPy_BMEditSelSeq *self, Py_ssize_t keynum)
 {
   BMEditSelection *ese;