Fix sorting of rows with different length using custom itemgetter

[arturponinski]

This PR has the objective of fixing sorting of rows with different length. Closes https://github.com/petl-developers/petl/issues/385

Changes

1. Changed comparable_itemgetter to fallback to a custom implementation in case of IndexError and KeyError

Checklist

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• [ ] Source Code rules apply:
  • [x] Includes unit tests
  • [ ] New functions have docstrings with examples that can be run with doctest
  • [ ] New functions are included in API docs
  • [ ] Docstrings include notes for any changes to API or behaviour
  • [x] All changes are documented in docs/changes.rst
• [ ] Versioning and history tracking rules apply:
  • [x] Using atomic commits when possible
  • [x] Commits are reversible when possible
  • [x] There is no incomplete changes in the pull request
  • [x] There is no accidental garbage added in source code
• [ ] Testing rules apply:
  • [ ] Tested locally using tox / pytest
  • [ ] Automated testing passes (see CI)
  • [ ] Unit test coverage has not decreased (see Coveralls)
• [ ] State of these changes is:
  • [ ] Just a proof of concept
  • [x] Work in progress / Further changes needed
  • [ ] Ready to review
  • [ ] Ready to merge

[coveralls]

Pull Request Test Coverage Report for Build 1813523682

• 28 of 28 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 91.871%

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Totals
Change from base Build 1813505534:	0.02%
Covered Lines:	12454
Relevant Lines:	13556

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💛 - Coveralls