[Python3] TokenStreamRewriter: Interval missing start and stop fields

In TokenStreamRewriter, the function getText() tries to access fields start and stop of provided interval argument
```
start = interval.start
stop = interval.stop

```
However, it seems that interval is an ordinary tuple. Therefore, what works is

```
start = interval[0]
stop = interval[1]

```

I also checked in the rest of the codebase, and the suggested fix was done in e.g. BufferedTokenStream.
Does this change make sense?

Start in the Python3 TokenStreamRewriter implementation and inspect getText(), then compare its interval handling with BufferedTokenStream, where tuple indexing is already used. Verify that getText() accepts ordinary tuple intervals and that the relevant existing tests, if located, still pass.

Written by the indexing model from the issue text.