Fix: bt2: remove circular import (not supported before Python 3.5)

Fix: bt2: remove circular import (not supported before Python 3.5)

This fixes the bindings failing to load with a circular import error on
SLES12 with Python 3.4:

 Traceback (most recent call last):
   File "/usr/lib64/python3.4/unittest/loader.py", line 323, in _find_tests
     module = self._get_module_from_name(name)
   File "/usr/lib64/python3.4/unittest/loader.py", line 301, in _get_module_from_name
     __import__(name)
   File "/home/mjeanson/Git/babeltrace/tests/bindings/python/bt2/test_clock_class.py", line 22, in <module>
     import bt2
   File "/home/mjeanson/Git/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/__init__.py", line 26, in <module>
     from bt2.clock_class import *
   File "/home/mjeanson/Git/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/clock_class.py", line 23, in <module>
     from bt2 import native_bt, object, utils
   File "/home/mjeanson/Git/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/utils.py", line 24, in <module>
     import bt2.logging
   File "/home/mjeanson/Git/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/logging.py", line 23, in <module>
     from bt2 import native_bt, object, utils
 ImportError: cannot import name 'utils'

Jonathan Rajotte investigated why this happens with Python 3.4:

This is due to a circular import. utils import logging, logging import
utils.

The proposed solution is valid.

PDB callstack leading to error:
 -> tests = loader.discover(start_dir, pattern)
   /usr/lib64/python3.4/unittest/loader.py(275)discover()
 -> tests = list(self._find_tests(start_dir, pattern))
   /usr/lib64/python3.4/unittest/loader.py(323)_find_tests()
 -> module = self._get_module_from_name(name)
   /usr/lib64/python3.4/unittest/loader.py(301)_get_module_from_name()
 -> __import__(name)
   <frozen importlib._bootstrap>(2237)_find_and_load()
   <frozen importlib._bootstrap>(2226)_find_and_load_unlocked()
   <frozen importlib._bootstrap>(1200)_load_unlocked()
   <frozen importlib._bootstrap>(1129)_exec()
   <frozen importlib._bootstrap>(1471)exec_module()
   <frozen importlib._bootstrap>(321)_call_with_frames_removed()
   /home/jenkins/babeltrace/tests/plugins/src.ctf.fs/query/test_query_trace_info.py(21)<module>()
 -> import bt2
   <frozen importlib._bootstrap>(2237)_find_and_load()
   <frozen importlib._bootstrap>(2226)_find_and_load_unlocked()
   <frozen importlib._bootstrap>(1200)_load_unlocked()
   <frozen importlib._bootstrap>(1129)_exec()
   <frozen importlib._bootstrap>(1471)exec_module()
   <frozen importlib._bootstrap>(321)_call_with_frames_removed()
   /home/jenkins/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/__init__.py(26)<module>()
 -> from bt2.clock_class import *
   <frozen importlib._bootstrap>(2237)_find_and_load()
   <frozen importlib._bootstrap>(2226)_find_and_load_unlocked()
   <frozen importlib._bootstrap>(1200)_load_unlocked()
   <frozen importlib._bootstrap>(1129)_exec()
   <frozen importlib._bootstrap>(1471)exec_module()
   <frozen importlib._bootstrap>(321)_call_with_frames_removed()
   /home/jenkins/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/clock_class.py(23)<module>()
 -> from bt2 import native_bt, object, utils
   <frozen importlib._bootstrap>(2284)_handle_fromlist()
   <frozen importlib._bootstrap>(321)_call_with_frames_removed()
  <frozen importlib._bootstrap>(2237)_find_and_load()
   <frozen importlib._bootstrap>(2226)_find_and_load_unlocked()
   <frozen importlib._bootstrap>(1200)_load_unlocked()
   <frozen importlib._bootstrap>(1129)_exec()
   <frozen importlib._bootstrap>(1471)exec_module()
   <frozen importlib._bootstrap>(321)_call_with_frames_removed()
   /home/jenkins/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/utils.py(24)<module>()
 -> import bt2.logging
   <frozen importlib._bootstrap>(2237)_find_and_load()
   <frozen importlib._bootstrap>(2226)_find_and_load_unlocked()
   <frozen importlib._bootstrap>(1200)_load_unlocked()
   <frozen importlib._bootstrap>(1129)_exec()
   <frozen importlib._bootstrap>(1471)exec_module()
   <frozen importlib._bootstrap>(321)_call_with_frames_removed()
 > /home/jenkins/babeltrace/src/bindings/python/bt2/build/build_lib/bt2/logging.py(23)<module>()
 -> from bt2 import native_bt, object, utils

This can be reproduce easily on python 3.4.6 using pyenv and the following reproducer:

 (my-virtual-env-3.4.6) joraj@/tmp/test[]$ tree
 .
 ├── reprod
 │   ├── __init__.py
 │   ├── pkg1.py
 │   ├── pkg2.py
 │   └── start.py
 └── test.py

test.py:
 import reprod
 print("got here")

__init__.py:
 __version__= '1.0.0'
 from reprod.start import *

pkg1.py:
 from reprod import pkg2

pkg2.py:
 from reprod import pkg1

start.py:
 from reprod import pkg1

Using Python 3.4.6:

 (my-virtual-env-3.4.6) joraj@/tmp/test[]$ python --version
 Python 3.4.6
 (my-virtual-env-3.4.6) joraj@/tmp/test[]$ python test.py
 Traceback (most recent call last):
   File "test.py", line 1, in <module>
     import reprod
   File "/tmp/test/reprod/__init__.py", line 3, in <module>
     from reprod.start import *
   File "/tmp/test/reprod/start.py", line 1, in <module>
     from reprod import pkg1
   File "/tmp/test/reprod/pkg1.py", line 1, in <module>
     from reprod import pkg2
   File "/tmp/test/reprod/pkg2.py", line 1, in <module>
     from reprod import pkg1
 ImportError: cannot import name 'pkg1'
 (my-virtual-env-3.4.6) joraj@/tmp/test[]$

The same reproducer run with python 3.5.6 works fine (again using pyenv):

 (my-virtual-env-3.5.6) joraj@/tmp/test[]$ python --version
 Python 3.5.6
 (my-virtual-env-3.5.6) joraj@/tmp/test[]$ python test.py
 got here
 (my-virtual-env-3.5.6) joraj@/tmp/test[]$

This is mostly due to the work done regarding circular dependency import
involving relative import[1][2].

[1] https://docs.python.org/3.5/whatsnew/3.5.html#other-language-changes
[2] https://bugs.python.org/issue17636

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: If43a6f9fad3d2b082fcd912f9ff6c193537d9f77