bts_osmotrx: Fix mkdtemp use inf python < 3.5

We are currently seeing this issues in prod setup, which uses python
3.4:

  File "/home/jenkins/workspace/osmo-gsm-tester_run/osmo-gsm-tester/src/osmo_gsm_tester/bts_osmotrx.py", line 44, in __init__
    self.pcu_sk_tmp_dir = tempfile.mkdtemp(None, 'ogtpcusk', None)
  File "/usr/lib/python3.4/tempfile.py", line 432, in mkdtemp
    file = _os.path.join(dir, prefix + name + suffix)
TypeError: Can't convert 'NoneType' object to str implicitly

Acording to mkdtemp man in python3:
"Changed in version 3.5: suffix, prefix, and dir may now be supplied in
bytes in order to obtain a bytes return value. Prior to this, only str
was allowed. suffix and prefix now accept and default to None to cause
an appropriate default value to be used."

This issue went unnoticed because the RnD env is running debian 9, which
probably has python version >= 3.5.

Change-Id: If3226183fe21b4820149b1d03207193dc6f139e2
1 file changed