Depend on ipython_genutils to fix issue with missing ipykernel dependency

[microbit-carlos]

We keep ipykernel below version 6 to keep compatibility with macOS 10.13 (and possibly 10.14 as well), since v6 includes a dependency on debugpy and that only has wheels for newer macOS versions. This also maintains compatibility with Python 3.6, as v6 is Python 3.7+.

Turns out v5 of ipykernel depends on ipython_genutils, but this isn't explicitly declared as a dependency in the package. It also depends on traitlets, which incidentally brought ipython_genutils, so it's worked fine so far. However traitlets dropped this dependency in v5.1, so ipython_genutils isn't picked any more and as a workaround we need to manually specify it in the Mu dependencies and the user venv dependencies.

More info in #1840 and ipython/ipykernel#759.

Fixes #1840, fixes #1825, and fixes #1816.

[carlosperate]

@tmontes I think you mentioned it might be enough to add this dependency only to the user virtualenv, but I've added it to setup.py just in case as well.

[carlosperate]

Merging quickly to be able to create working builds again.

[tmontes]

@tmontes I think you mentioned it might be enough to add this dependency only to the user virtualenv, but I've added it to setup.py just in case as well.

...ok, should be harmless (crossing fingers!). Things are looking good on my side:

• Cleaned up everything (dev + user venvs).
• Updated master.
• Created dev venv from scratch + pip installed Mu onto it.
• Mu launched and ran fine from source.
• Packaged with pup, installed, Mu launched ran fine.
• Both on macOS and Windows.

Go @ntoll! :)