DEPR/TEST: inspect deprecation warning
hmm, this looks pretty new. should fix this code.

```
...../home/travis/build/pydata/pandas/pandas/tests/test_panel.py:201: DeprecationWarning: inspect.getargspec() is deprecated, use inspect.signature() instead
```

build is [here](https://travis-ci.org/pydata/pandas/jobs/103526993). This is a numpy-dev build, need to check out the other builds.
