Add explicit encoding in open calls

[dlukes]

Having enabled PYTHONWARNDEFAULTENCODING on Python 3.10, I now get yelled at (which is a good thing!) whenever Python opens a file in text mode without an encoding specified, implicitly falling back on the locale. It just happened when calling jupyter server --generate-config, so I took a bit of time to add explicit encoding arguments to open calls in jupyter_core.

Please review if UTF-8 is the appropriate encoding in all cases, I'm not otherwise familiar with the codebase, so it's not always easy for me to see. Most of the calls seem to handle Python / CSS / JS files, where UTF-8 should be appropriate, but one of the functions I'm not entirely sure about is paths.secure_write, which can probably be used to write lots of different files (though that in turn means it can be used to write Python files, which should be UTF-8).

[vidartf]

This looks like a clear improvement to me.

[jasongrout]

Looks good to me too. Thanks!