Make NDArrayType.__len__() a method, not a property

[jdavies-st]

Fixes #671.

[stsci-bot]

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[stsci-bot]

Hi there @jdavies-st 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

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[codecov]

Codecov Report

Merging #673 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   93.29%   93.34%   +0.04%     
==========================================
  Files          39       39              
  Lines        4295     4295              
==========================================
+ Hits         4007     4009       +2     
+ Misses        288      286       -2

Impacted Files	Coverage Δ
asdf/tags/core/ndarray.py	93.65% <ø> (+0.52%)	⬆️

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[drdavella]

@jdavies-st this looks good! The one minor nit is I think we should add a v2.3.4 section to the change log since this is a bug. If it eventually winds up just being released with v2.4.0, we'll fix the change log later.