[gdb/dap] Fix regressions with python 3.6

With test-case gdb.dap/ada-arrays.exp, on Leap openSUSE 15.6 with python 3.6,
I run into:
...
Python Exception <class 'TypeError'>: 'type' object is not subscriptable
Error occurred in Python: 'type' object is not subscriptable
ERROR: tcl error sourcing ada-arrays.exp.
...

This is due to using a python 3.9 construct:
...
thread_ids: dict[int, int] = {}
...

Fix this by using typing.Dict instead.

Tested on x86_64-linux.

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