skip test if ipykernel not there.

This is in particular useful for publishing on conda-forge when bumping
Python version, as ipykernel and ipython have circular dependencies when
running the full test suite.

.travis.yml

@@ -103,7 +103,7 b' matrix:'
       python: "nightly"
       dist: xenial
     - arch: amd64
-      python: "3.9-dev"
+      python: "3.9"
     - os: osx
       language: generic
       python: 3.7

IPython/core/tests/test_display.py

@@ -138,7 +138,9 b' def _get_inline_config():'
     from ipykernel.pylab.config import InlineBackend
     return InlineBackend.instance()
 
[e-mail]_without('matplotlib')
+
[e-mail]_without("ipykernel")
[e-mail]_without("matplotlib")
 def test_set_matplotlib_close():
     cfg = _get_inline_config()
     cfg.close_figures = False
@@ -173,7 +175,9 b' def test_set_matplotlib_formats():'
             else:
                 nt.assert_not_in(Figure, f)
 
[e-mail]_without('matplotlib')
+
[e-mail]_without("ipykernel")
[e-mail]_without("matplotlib")
 def test_set_matplotlib_formats_kwargs():
     from matplotlib.figure import Figure
     ip = get_ipython()