DEPR/TEST: inspect deprecation warning

[jreback]

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. This is a numpy-dev build, need to check out the other builds.

[jreback]

closed by #12102