BUG/BLD: pytest not a runtime dep
on master - now that we've exposed a testing api, need to wrap the import of pytest

```
>>> import pandas

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\Chris\Anaconda\envs\py361\lib\site-packages\pandas\__init__.py", line 60, in <module>
    import pandas.testing
  File "C:\Users\Chris\Anaconda\envs\py361\lib\site-packages\pandas\testing.py", line 7, in <module>
    from pandas.util.testing import (
  File "C:\Users\Chris\Anaconda\envs\py361\lib\site-packages\pandas\util\testing.py", line 22, in <module>
    import pytest
ModuleNotFoundError: No module named 'pytest'
```