fix constant folding of attributes

[charles-cooper]

What I did

Fix #2329

How I did it

Relax a restriction on when constant folding can happen

How to verify it

See the test / compile the example in #2329

Description for the changelog

Allow constant folding of

.balance

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[codecov-commenter]

Codecov Report

Merging #2533 (75b24d1) into master (9a5dd50) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2533      +/-   ##
==========================================
+ Coverage   84.67%   84.70%   +0.02%     
==========================================
  Files          93       93              
  Lines        9301     9304       +3     
  Branches     2096     2095       -1     
==========================================
+ Hits         7876     7881       +5     
+ Misses        940      939       -1     
+ Partials      485      484       -1     

Impacted Files	Coverage Δ
vyper/ast/folding.py	95.65% <ø> (+2.03%)	⬆️
vyper/old_codegen/external_call.py	94.33% <0.00%> (ø)
vyper/builtin_functions/functions.py	88.62% <0.00%> (ø)
vyper/old_codegen/parser_utils.py	83.01% <0.00%> (+0.23%)	⬆️

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