feat: remove Index AST node

[tserg]

What I did

Deprecate Index AST node.

How I did it

How to verify it

Commit message

remove Index AST node type, it has been deprecated from the python AST
since python3.9.

Description for the changelog

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[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (db8ac3c) 84.78% compared to head (8fe6e7d) 84.75%.
Report is 1 commits behind head on master.

❗ Current head 8fe6e7d differs from pull request most recent head a99e541. Consider uploading reports for the commit a99e541 to get more accurate results

Files	Patch %	Lines
vyper/semantics/types/subscriptable.py	60.00%	0 Missing and 2 partials ⚠️
vyper/codegen/expr.py	66.66%	1 Missing ⚠️
vyper/semantics/analysis/utils.py	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3757      +/-   ##
==========================================
- Coverage   84.78%   84.75%   -0.04%     
==========================================
  Files          92       92              
  Lines       13141    13128      -13     
  Branches     2930     2929       -1     
==========================================
- Hits        11142    11126      -16     
- Misses       1535     1539       +4     
+ Partials      464      463       -1     

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[charles-cooper]

lgtm, thanks!