Increase minimum jupyter_core dependency

[kevin-bates]

This bumps the minimum core dependency to 4.6.0 since that version of core is the first to include the is_hidden() method - on which the server relies. These dependencies had been inherited from Notebook and it defines is_hidden() locally so the previous core dependency (4.4.0) is still valid in that scenario.

Fixes #496

[codecov-commenter]

Codecov Report

Merging #509 (1302c7e) into master (48cf6a5) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #509      +/-   ##
==========================================
+ Coverage   77.08%   77.14%   +0.06%     
==========================================
  Files         107      107              
  Lines        9490     9490              
  Branches     1026     1026              
==========================================
+ Hits         7315     7321       +6     
+ Misses       1807     1803       -4     
+ Partials      368      366       -2     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/handlers.py	62.96% <0.00%> (+1.58%)	⬆️

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[mwakaba2]

lgtm!