Prepare v1.5.1 release

[lognaturel]

• Make sure the version of ODK Validate in the repo is up-to-date:
• Run all tests through Validate
• Draft a new GitHub release with the list of merged PRs. Follow the title and description pattern of the previous release.
• Update CHANGES.txt with the text of the draft release.
• Update README.rst, setup.py, pyxform/init.py with the new release version number.

[codecov-commenter]

Codecov Report

Merging #537 (2278d05) into master (3184e9d) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 2278d05 differs from pull request most recent head 13ee243. Consider uploading reports for the commit 13ee243 to get more accurate results

@@           Coverage Diff           @@
##           master     #537   +/-   ##
=======================================
  Coverage   84.00%   84.00%           
=======================================
  Files          25       25           
  Lines        3732     3732           
  Branches      872      872           
=======================================
  Hits         3135     3135           
  Misses        452      452           
  Partials      145      145           

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

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[pbowen-oc]

Thank you!