Update avl_tree.py

[zky001]

it's true definition of AVL tree,change left and right rotation,and add avl_tree doctest

Describe your change:

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

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Travis tests have failed

Hey @zky001,
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[zky001]

Hey @zky001,
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This error is caused by flake8 W291 trailing whitespace，this whitespace is caused by my doctest's output

[zky001]

Travis tests have failed

Hey @zky001,
Please read the following log in order to understand the failure reason.
It'll be awesome if you fix what's wrong and commit the changes.

TravisBuddy Request Identifier: 4af64640-b43d-11ea-bd89-f57d85bb28d1

when I delete this whitespace in my doctest, this action will raise test failed error.

[cclauss]

"\n".join(line.rstrip() for line in my_doctest())

[cclauss]

Let's change the method traversale() --> __str__() and then change instances of t.traversale() into str(t).

[cclauss]

Also, please change isEmpty() to is_empty().

[zky001]

Also, please change isEmpty() to is_empty().

thanks a lot

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[cclauss]

Thanks for ALL the cleanup!