Prepare v1.5.1 release

[lognaturel]

• [x] Make sure the version of ODK Validate in the repo is up-to-date:
• [x] Run all tests through Validate
• [x] Draft a new GitHub release with the list of merged PRs. Follow the title and description pattern of the previous release.
• [x] Update CHANGES.txt with the text of the draft release.
• [x] 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%.

:exclamation: 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)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 3184e9d...13ee243. Read the comment docs.

[pbowen-oc]

Thank you!