fix: attach stderr to jupyter process

[joh]

Without stderr, kernels fail to start and JupyniumStartSync times out.

fixes #97

[joh]

Style should be fixed now :)

[kiyoon]

Thanks a lot for the contribution! I didn't know this would make a difference but it seems it can fix many problems.

I saw different behaviour when using JupyniumStartAndAttachToServerInTerminal. I assume if you use this command this problem doesn't exist?

[joh]

You're right, with JupyniumStartAndAttachToServerInTerminal it works without the fix, presumably because then stderr is (implicitly) attached to the neovim buffer.

As a comparison, here's /proc/$NOTEBOOK_PID/fd/2 for the two variants:

With JupyniumStartAndAttachToServer:
lrwx------ 1 joh joh 64 Oct 2 23:29 2 -> 'socket:[5855810]'

With JupyniumStartAndAttachToServerInTerminal:
lrwx------ 1 joh joh 64 Oct 2 23:27 2 -> /dev/pts/15

Now with the fix applied, we have:
With JupyniumStartAndAttachToServer:
lrwx------ 1 joh joh 64 Oct 2 23:34 2 -> '/tmp/tmpi0a_ynrx (deleted)'