Make iter_dirs and iter_files yield items in alphabetical order

[elemoine]

This PR makes the iter_dirs and iter_files functions yield their items in alphabetical order.

The tests/integration/test_files.py::test_iter_files test didn't pass for me because of that.

Successful PR Checklist:

• Tests Not relevant
• Documentation (optionally: run spell checking) Not relevant
• Had a good time contributing? (feel free to give some feedback) First contribs always feel good 😄

[codecov]

Codecov Report

Merging #56 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   56.35%   56.35%           
=======================================
  Files          14       14           
  Lines         598      598           
  Branches       81       81           
=======================================
  Hits          337      337           
  Misses        254      254           
  Partials        7        7

Flag	Coverage Δ
#integration	12.37% <100%> (ø)	⬆️
#unit	55.18% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
septentrion/files.py	86.79% <100%> (ø)	⬆️

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