5.6.0

New features:

• Execute cells by clicking icon in input prompt (#3535, #3687)
• New "Save as" menu option (#3289)
• When serving on a loopback interface, protect against DNS rebinding
  by checking the Host header from the browser (#3714). This check
  can be disabled if necessary by setting
  NotebookApp.allow_remote_access. (Disabled by default while we
  work out some Mac issues in #3754).
• Add kernel_info_timeout traitlet to enable restarting slow kernels
  (#3665)
• Add custom_display_host config option to override displayed URL
  (#3668)
• Add /metrics endpoint for Prometheus Metrics (#3490)
• Update to MathJax 2.7.4 (#3751)
• Update to jQuery 3.3 (#3655)
• Update marked to 0.4 (#3686)

Fixing problems:

• Don't duplicate token in displayed URL (#3656)
• Clarify displayed URL when listening on all interfaces (#3703)
• Don't trash non-empty directories on Windows (#3673)
• Include LICENSE file in wheels (#3671)
• Don't show "0 active kernels" when starting the notebook (#3696)

Testing:

• Add find replace test (#3630)
• Selenium test for deleting all cells (#3601)
• Make creating a new notebook more robust (#3726)

See the 5.6 milestone on GitHub for a complete list of pull
requests involved in this release.