Add option to use custom Outstream in kernel app

[eendebakpt]

Using a custom Outstream allows us to set the isatty option that was introduced in ipython/ipykernel#683. Setting the isatty to True allows packages like rich and colorama to have colored output in Spyder.

Also see jupyter/qtconsole#486

@ccordoba12

[peendebak]

@ccordoba12 I looked into the code of spyder-kernels, but could not find where the Outstream is constructed. Could you give a pointer?

[eendebakpt]

@ccordoba12 Could it be that the OutStream is created here: https://github.com/ipython/ipykernel/blob/36d6a91639b3ec09502b5454d30751ad81ffab27/ipykernel/inprocess/ipkernel.py#L155 ?

In that case we would require another PR to ipykernel to allow setting the isatty option

[real-yfprojects]

In this message @ccordoba12 pointed to the kernelapp.IPKernelApp class. I think the Outstream instance is created here:

https://github.com/ipython/ipykernel/blob/7ed57cb4a03e719074c373029cf9f2e6dfd2c6de/ipykernel/kernelapp.py#L421

Here the DottedObjectName in outsream_class is imported and then used as sys.stdout.

[eendebakpt]

@real-yfprojects @ccordoba12 Thanks for the hint. There are various ways to make sure spyder uses a different Outstream class. I opted for one without any changes to the upstream ipykernel and one with minimal patching of the original SpyderKernelApp. Could you have a look at #305