Make it possible to adjust the attributes used when comparing nodes.

[longhotsummer]

Subclasses can use this determine which attributes for a node should be considered when diffing. By default, they are all considered.

This makes it possible to ignore an attribute on a particular node when calculating diffs, without removing it from the XML tree before hand and having to add it back after the diff.

[longhotsummer]

Just to note that the tests are passing, but travis is failing because black is unhappy with the formatting of code that is not part of this PR.

[regebro]

What is the use case for this? This is, after all, a differ, it should show differences.

[longhotsummer]

I'm using xmldiff to compare XML documents which I'm then showing to the user. The documents are allowed to differ on some attributes. I don't want to remove them from the XML tree before diffing, since I still want to show the attributes, I simply want to ignore them for diff purposes.

This is a bit like git or GitHub's "ignore whitespace in diffs" feature: in some situations it helps the user to make sense of the differences, even though it's not strictly accurate.

I didn't want to add this entire feature to xmldiff, so I simply want to make it a bit easier for me (and others) to implement this.

[regebro]

OK. Could you rebase on master, I've fixed the Travis failinings.
Also please update the Changes.txt and add yourself to the contributors list.

[longhotsummer]

@regebro thanks -- rebased and added my name to the contributor list. The CHANGES.rst is also updated.

[coveralls]

Pull Request Test Coverage Report for Build 254

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0004%) to 99.793%

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Totals
Change from base Build 252:	0.0004%
Covered Lines:	963
Relevant Lines:	965

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💛 - Coveralls

[longhotsummer]

Thanks @regebro ! Do you think you will be able to release to pypi soon?