fix: keys suggested in the order they are defined

[andreoliwa]

Proposed changes

• Add https://github.com/ianlini/flatten-dict
• Add https://github.com/dpath-maintainers/dpath-python
• Replace DictBlender by dpath/flatten/unflatten
• Positive side effect: the suggestion of initial contents now displays the keys in the order they were defined in the TOML file, and not in alphabetical order anymore.
• Remove OrderedDict; dicts are already ordered in Python 3.7

Checklist

• Read the contribution guidelines
• Run make locally before pushing commits
• Add tests for the relevant parts:
  • API
  • CLI
  • flake8 plugin (normal mode)
  • flake8 plugin (offline mode)
• Write documentation when there's a new API or functionality

[coveralls]

Pull Request Test Coverage Report for Build 1700476773

• 77 of 78 (98.72%) changed or added relevant lines in 8 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 97.019%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/nitpick/blender.py	49	50	98.0%

Files with Coverage Reduction	New Missed Lines	%
src/nitpick/blender.py	1	96.68%

Totals
Change from base Build 1696026181:	-0.01%
Covered Lines:	1987
Relevant Lines:	2031

---

💛 - Coveralls

[github-actions]

🎉 This PR is included in version 0.31.0 🎉

The release is available on:

• v0.31.0
• GitHub release

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