Added AVL Tree [Python]

[Jatin-Goyal-552]

Fixes #724

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[theyashshahs]

@Jatin-Goyal-552 fill the checkboxes in PR body and change the PR title

[theyashshahs]

@Jatin-Goyal-552 mention the Fixes #{ISSUE_NUMBER} that is fixed in this PR

[Jatin-Goyal-552]

Sir,I am not able to mention issue number in this PR.

[theyashshahs]

Sir,I am not able to mention issue number in this PR.

What is the issue in that just mention the issue number you were assigned to in place of {ISSUE_NUMBER}