🧪 TESTS: add ipykernel to dependencies (#70)

ipykernel must have been dropped from one of the other dependencies,
since it is no longer installed in the test environment.

Author: chrisjsewell

setup.py

@@ -47,6 +47,7 @@
         "cli": ["click", "click-completion", "click-log", "tabulate", "pyyaml"],
         "code_style": ["flake8<3.8.0,>=3.7.0", "black", "pre-commit==1.17.0"],
         "testing": [
+            "ipykernel",
             "coverage",
             "pytest>=3.6,<4",
             "pytest-cov",