For Fletcher32, define a constant FOOTER_LENGTH

[jakirkham]

Replace the hard-coding of 4 in Fletcher32 by defining a constant using the sizeof the value placed in the footer of the encoded result. Then use this constant throughout. Makes its usage a little clear to code readers. Also minimize the chance the wrong value accidentally slips in somewhere.

---

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.96%. Comparing base (27aeda2) to head (fcf3221).
Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #718   +/-   ##
=======================================
  Coverage   99.96%   99.96%           
=======================================
  Files          63       63           
  Lines        2771     2771           
=======================================
  Hits         2770     2770           
  Misses          1        1           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[d-v-b]

thanks @jakirkham !