refactor: for loop target parsing

[charles-cooper]

What I did

use a parser trick to get always-correct source locations into for-loop target annotations.

follow-up to #3721; reverts the AnnAssign pre-parser change from there.

How I did it

see commit

How to verify it

check new test will not pass without the parser changes.

Commit message

use a parser trick to get always-correct source locations into for-loop
target annotations.

this fixes an issue with parsing the for loop target (introduced in
ddfce5273b3), where the target annotation is not decorated with the
correct line/column offsets. a6dc432db2df partially fixed the issue, but
does not decorate correctly when the annotation is split across multiple
lines. this commit approaches the problem differently, by using
`untokenize()` to produce a source code for the annotation during AST
massaging which "happens" to have the annotation in exactly the same
place as it appeared in the original source.

small refactors:
- move annotate_python_ast to be earlier in the file. it's somehow
  easier to navigate to it when it comes right before the definition of
  AnnotatingVisitor.
- revert changes from a6dc432db2df to ast/pre_parser.py; go back to
  slurping just the annotation as opposed to the target+annotation
  together.

Description for the changelog

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a6dc432) 84.11% compared to head (4dcb201) 84.16%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3724      +/-   ##
==========================================
+ Coverage   84.11%   84.16%   +0.05%     
==========================================
  Files          92       92              
  Lines       13123    13106      -17     
  Branches     2926     2922       -4     
==========================================
- Hits        11038    11031       -7     
+ Misses       1663     1656       -7     
+ Partials      422      419       -3     

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[tserg]

This is truly black magic.

I tried it with a nested error and it works too:

@external
def foo():
    for i: DynArray[\
        uint256, 3] in [[1], [2], [3]]:
        for j: \
            uint247 in i:
            pass

vyper.exceptions.UnknownType: No builtin or user-defined type named 'uint247'. Did you mean 'uint24', or maybe 'uint240'?
  contract "tmp/old_for3.vy:7", function "foo", line 7:12 
       6         for j: \
  ---> 7             uint247 in i:
  -------------------^
       8