Make lldbDataFormatters.py compatible with Python 3.8 (llvm#82518)

I just tried to load this into LLDB built against Python 3.8.5 and got
the following error: `TypeError: 'type' object is not subscriptable`. I
could fix this by wrapping the annotations in quotes but since Python
3.7 this syntax can be enabled with `from __future__ import
annotations`.

llvm/utils/lldbDataFormatters.py

@@ -3,6 +3,7 @@
 
 Load into LLDB with 'command script import /path/to/lldbDataFormatters.py'
 """
+from __future__ import annotations
 
 import collections
 import lldb