tests/thread/thread_stacksize1.py: Increase stack size for CPython.

On arm64 with CPython:

  >>> _thread.stack_size(32*1024)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ValueError: size not valid: 32768 bytes

So increase the CPython value in the test to 512k so it runs on more
systems (on modern Linux the default stack size is usually 8MB).

tests/thread/thread_stacksize1.py

@@ -9,7 +9,7 @@
 if sys.implementation.name == "micropython":
     sz = 2 * 1024
 else:
-    sz = 32 * 1024
+    sz = 512 * 1024
 
 
 def foo():