Normalise os_path

[krassowski]

Port of jupyter-server/jupyter_server#886, fixes #6458

[krassowski]

Hi, could someone take a look at this and #6461? @echarles @RRosio? It fixes a regression preventing users from copying files (unless they enabled hidden files access).

[echarles]

LGTM Thx @krassowski

[echarles]

I have merged:

• Normalise os_path #6540 on 6.5.x branch: do you need it also on 6.4.x branch?
• Fix a typo in exception text #6461 on 6.4.x branch: reverse question, do you need something similar on 6.5.x branch? Or should that change go to nbclassic?

Also the intent was to not release any new 6.4.x and invite users to migrate to 6.5. As we are still looking for feedbacks on 6.5 (see this blog post), I guess there is still room to cut a new 6.4.x.

[krassowski]

I don't feel it is critical to backport these fixes to 6.4.x if 6.5.0 goes smoothly. I opened #6545 to forward port #6461.