Failure during pytest (and solution for python3)
Author: dandelin
Pull request: no

```
foo@bar:~/foo/bar/pytorch-pretrained-BERT$ pytest -sv ./tests/
===================================================================================================================== test session starts =====================================================================================================================
platform linux -- Python 3.6.6, pytest-3.9.1, py-1.7.0, pluggy-0.8.0 -- /home/foo/.pyenv/versions/anaconda3-5.1.0/bin/python
cachedir: .pytest_cache
rootdir: /data1/users/foo/bar/pytorch-pretrained-BERT, inifile:
plugins: remotedata-0.3.0, openfiles-0.3.0, doctestplus-0.1.3, cov-2.6.0, arraydiff-0.2, flaky-3.4.0
collected 0 items / 3 errors

=========================================================================================================================== ERRORS ============================================================================================================================
___________________________________________________________________________________________________________ ERROR collecting tests/modeling_test.py ___________________________________________________________________________________________________________
ImportError while importing test module '/data1/users/foo/bar/pytorch-pretrained-BERT/tests/modeling_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/modeling_test.py:25: in <module>
    import modeling
E   ModuleNotFoundError: No module named 'modeling'
_________________________________________________________________________________________________________ ERROR collecting tests/optimization_test.py _________________________________________________________________________________________________________
ImportError while importing test module '/data1/users/foo/bar/pytorch-pretrained-BERT/tests/optimization_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/optimization_test.py:23: in <module>
    import optimization
E   ModuleNotFoundError: No module named 'optimization'
_________________________________________________________________________________________________________ ERROR collecting tests/tokenization_test.py _________________________________________________________________________________________________________
ImportError while importing test module '/data1/users/foo/bar/pytorch-pretrained-BERT/tests/tokenization_test.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
tests/tokenization_test.py:22: in <module>
    import tokenization
E   ModuleNotFoundError: No module named 'tokenization'
===Flaky Test Report===


===End Flaky Test Report===
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Interrupted: 3 errors during collection !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
=================================================================================================================== 3 error in 0.60 seconds ==================================================================================================================
```

In python 3, `python -m pytest -sv tests/` works fine.

Comments:
Thanks, I update the readme.