Silence some writable-string warnings

Cast string constants explicitly to (char*) since the
method calls aren't going to modify the strings.

Author: jkseppan

src/file_compat.h

@@ -192,15 +192,15 @@ mpl_PyFile_OpenFile(PyObject *filename, const char *mode)
     if (open == NULL) {
         return NULL;
     }
-    return PyObject_CallFunction(open, "Os", filename, mode);
+    return PyObject_CallFunction(open, (char*)"Os", filename, mode);
 }
 
 static NPY_INLINE int
 mpl_PyFile_CloseFile(PyObject *file)
 {
     PyObject *ret;
 
-    ret = PyObject_CallMethod(file, "close", NULL);
+    ret = PyObject_CallMethod(file, (char*)"close", NULL);
     if (ret == NULL) {
         return -1;
     }

src/ft2font.cpp

@@ -2208,7 +2208,7 @@ FT2Font::make_open_args(PyObject *py_file_arg, FT_Open_Args *open_args)
         open_args->stream = &stream;
     } else {
         if (PyObject_HasAttrString(py_file_arg, "read") &&
-            (data = PyObject_CallMethod(py_file_arg, "read", ""))) {
+            (data = PyObject_CallMethod(py_file_arg, (char*)"read", (char*)""))) {
             if (PyBytes_AsStringAndSize(data, &data_ptr, &data_len)) {
                 goto exit;
             }