[MRG] deprecate 'dump' command

[ctb]

Fixes #830

Note: this is a PR into the stable branch, because we are deprecating something in 3.x and then removing in 4.0.

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (stable@2093b99). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             stable    #1147   +/-   ##
=========================================
  Coverage          ?   92.43%           
=========================================
  Files             ?       73           
  Lines             ?     5695           
  Branches          ?        0           
=========================================
  Hits              ?     5264           
  Misses            ?      431           
  Partials          ?        0           

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[ctb]

ready to review & merge!