feat: require loop variable annotation

[tserg]

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Codecov Report

Merging #3544 (36ef266) into master (cfda16c) will decrease coverage by 33.58%.
The diff coverage is 4.00%.

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@@             Coverage Diff             @@
##           master    #3544       +/-   ##
===========================================
- Coverage   89.11%   55.53%   -33.58%     
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  Files          85       85               
  Lines       11362    11373       +11     
  Branches     2584     2588        +4     
===========================================
- Hits        10125     6316     -3809     
- Misses        816     4527     +3711     
- Partials      421      530      +109     

Files Changed	Coverage Δ
vyper/codegen/stmt.py	32.55% <0.00%> (-57.37%)	⬇️
vyper/semantics/analysis/local.py	33.24% <0.00%> (-58.38%)	⬇️
vyper/ast/pre_parser.py	38.83% <9.09%> (-45.95%)	⬇️

... and 66 files with indirect coverage changes

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[charles-cooper]

the issue with this is that it will conflict with iterator syntax if we ever support things like for x, y in zip(xs, ys): .... i think the thing to do here is maybe actually add more metadata in the pre_parser regarding the type annotation.