WeakMap support for Python

The documentation page for `haxe.ds.WeakMap` [here](https://api.haxe.org/haxe/ds/WeakMap.html) says it is available on all platforms, but when I call `new WeakMap()` on Python, I get this error:

> Traceback (most recent call last):
File "test.py", line 4941, in
sys_thread_Thread.threads = haxe_ds_WeakMap()
File "test.py", line 1395, in __init__
raise haxe_exceptions_NotImplementedException("Not implemented for this plat
form",None,_hx_AnonObject({'fileName': "haxe/ds/WeakMap.hx", 'lineNumber': 39, '
className': "haxe.ds.WeakMap", 'methodName': "new"}))
__main__.haxe_exceptions_NotImplementedException: Not implemented for this platf
orm

Either the docs should reflect this, or WeakMap should be implemented for Python.