[lldb] Make socket_packet_pump.py work in Python3

Summary:

The `string_escape` encoding used here was removed in Python 3 which makes the
test crash during tearDown:

```
  File "lldb/third_party/Python/module/unittest2/unittest2/case.py", line 386, in run
    self.tearDown()
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/gdbremote_testcase.py", line 124, in tearDown
    self._pump_queues.verify_queues_empty()
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/socket_packet_pump.py", line 55, in verify_queues_empty
    _dump_queue(self.packet_queue())
  File "lldb/packages/Python/lldbsuite/test/tools/lldb-server/socket_packet_pump.py", line 28, in _dump_queue
    print(codecs.encode(the_queue.get(True), "string_escape"))
LookupError: unknown encoding: string_escape
```

Just replace it with `repr` which should work in both Python versions.

Reviewers: labath, JDevlieghere

Reviewed By: labath, JDevlieghere

Subscribers: JDevlieghere

Differential Revision: https://reviews.llvm.org/D84017