Skip tests for recent ipykernel, and add check for IPython version

[mdickinson]

This PR:

• raises ImportError when attempting to import IPython-related machinery if ipykernel version 6 or later is installed or IPython version 8 or later is installed
• skips IPython-related tests when run in an environment containing either ipykernel version 6 or later or IPython version 8 or later

Closes #456.

Note: PR was updated to also check the version of IPython: it turns out that IPython >= 8 is also problematic. I've updated the description above.

[mdickinson]

It looks as though the IPython-related failures are occurring in spite of the ipykernel < 6 restriction. It seems likely that that restriction is not enough.

Comparison of installed packages between the last successful run of the Test with PyPI workflow and the above one:

Last successful run:

Successfully installed apptools-5.1.0 backcall-0.2.0 configobj-5.0.6 decorator-5.1.0 entrypoints-0.3 envisage-6.1.0.dev1 importlib-metadata-4.9.0 importlib-resources-5.4.0 ipykernel-5.5.6 ipython-7.30.1 ipython-genutils-0.2.0 jedi-0.18.1 jupyter-client-7.1.0 jupyter-core-4.9.1 matplotlib-inline-0.1.3 nest-asyncio-1.5.4 parso-0.8.3 pexpect-4.8.0 pickleshare-0.7.5 prompt-toolkit-3.0.24 ptyprocess-0.7.0 pyface-7.3.0 pygments-2.10.0 python-dateutil-2.8.2 pyzmq-22.3.0 six-1.16.0 tornado-6.1 traitlets-5.0.5 traits-6.3.2 traitsui-7.2.1 wcwidth-0.2.5 zipp-3.6.0

This run:

Successfully installed apptools-5.1.0 asttokens-2.0.5 backcall-0.2.0 black-21.12b0 click-8.0.3 configobj-5.0.6 decorator-5.1.1 entrypoints-0.3 envisage-6.1.0.dev1 executing-0.8.2 importlib-metadata-4.10.1 importlib-resources-5.4.0 ipykernel-5.5.6 ipython-8.0.1 ipython-genutils-0.2.0 jedi-0.18.1 jupyter-client-7.1.2 jupyter-core-4.9.1 matplotlib-inline-0.1.3 mypy-extensions-0.4.3 nest-asyncio-1.5.4 parso-0.8.3 pathspec-0.9.0 pexpect-4.8.0 pickleshare-0.7.5 platformdirs-2.4.1 prompt-toolkit-3.0.24 ptyprocess-0.7.0 pure-eval-0.2.2 pyface-7.3.0 pygments-2.11.2 python-dateutil-2.8.2 pyzmq-22.3.0 six-1.16.0 stack-data-0.1.4 tomli-1.2.3 tornado-6.1 traitlets-5.0.5 traits-6.3.2 traitsui-7.2.1 typing-extensions-4.0.1 wcwidth-0.2.5 zipp-3.7.0

[mdickinson]

Updated the code to also check for the IPython version. IPython 8 is currently causing issues.

[mdickinson]

Sorry, @rahulporuri: I changed the code in this PR substantially after your review. (It turned out that the IPython version also needed to be considered.) A re-review would be welcome, but there's no urgency.