Fix Python 3 issues related to OS plugins.

* lldb::tid_t was being converted incorrectly, so this is updated to use
PythonInteger instead of manual Python Native API calls.
* OSPlugin_RegisterContextData was assuming that the result of
  get_register_data was a string, when in fact it is a bytes.  So this
  method is updated to use PythonBytes to do the work.

llvm-svn: 257398