fix interface list in for

[charles-cooper]

What I did

following code was failing:

xs: MyInterface[N]
for x in self.xs:
    ...

How I did it

there was a missing case in a helper function

How to verify it

see tests

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2699 (9a78cff) into master (d4b1e3f) will decrease coverage by 0.84%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2699      +/-   ##
==========================================
- Coverage   86.54%   85.69%   -0.85%     
==========================================
  Files          91       91              
  Lines        9849     9852       +3     
  Branches     2479     2480       +1     
==========================================
- Hits         8524     8443      -81     
- Misses        829      905      +76     
- Partials      496      504       +8     

Impacted Files	Coverage Δ
vyper/codegen/types/convert.py	87.87% <100.00%> (+0.78%)	⬆️
vyper/semantics/types/__init__.py	100.00% <100.00%> (ø)
vyper/builtin_functions/functions.py	82.28% <0.00%> (-6.09%)	⬇️
vyper/utils.py	81.16% <0.00%> (-4.55%)	⬇️
vyper/semantics/types/value/numeric.py	83.80% <0.00%> (-3.81%)	⬇️
vyper/ast/nodes.py	92.49% <0.00%> (-1.65%)	⬇️
vyper/codegen/expr.py	78.17% <0.00%> (-1.33%)	⬇️

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