Prepare 3.0.0

[MattHJensen]

This prepares 3.0.0.

Pending resolution of #2450.

@Peter-Metz, could you look this over if you have an oportunity?

[MattHJensen]

Thanks to contributions from @Peter-Metz, @hdoupe, @MaxGhenis, @jdebacker, @chusloj, @andersonfrailey, many of which but not all are recognized in docs/about/releases.md!

[codecov]

Codecov Report

Merging #2474 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2474   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          13       13           
  Lines        2582     2582           
=======================================
  Hits         2580     2580           
  Misses          2        2           

Impacted Files	Coverage Δ
taxcalc/__init__.py	100.00% <100.00%> (ø)

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[Peter-Metz]

@MattHJensen this is looking good! Two minor comments in addition to the notes I left:

• Can drop the reference to CHANGES.md from the first line of RELEASING.md
• Will the parameter docs build when 3.0 is released? I'm not sure of the resolution of Add action for setting up parameters docs #2450

[MattHJensen]

I'll look at this JB failure later today. May have something to do with #2476 given that latest commit to this PR was rather innocuous.

[hdoupe]

@MattHJensen this looks like a flaky failure due to the anaconda service being unavailable temporarily:

I just restarted the tests.