[lldb] fix Python 3.9+ specific typing annotations (#156868)
This patch replaces `list` with its `typing` implementation, i.e
`list[str]` becomes `List[str]`.
[Type hinting generic in the standard collection were introduced in
Python 3.9](https://peps.python.org/pep-0585/), however the minimum
supported Python version for lldb is 3.8. This patch will unblock the
[bots for Ubuntu
20.04](https://ci.swift.org/view/Swift%20rebranch/job/oss-swift-rebranch-package-ubuntu-20_04/2847/consoleText),
which run on Python 3.8.