fix: with item precedence for multiple context managers with brackets

[Hellebore]

Currently when parsing code like the following:

with (open('d') as d,
      open('e') as e):
  f

tree-sitter is converting it into a with_statement which has one with_item, which contains a tuple of as_pattern.

In the Python specification multiple context managers should each be represented by a WithItem, each of which has a single (possibly aliased) context manager.

This was causing problems for us when using tree-sitter to parse such structures into an AST.

This PR adjusts (correctly I hope, though open to being wrong!) the precedence of the with_item to fix this issue and bring it into line with the Python AST.

[tausbn]

I think this looks reasonable. 👍

I was curious to see how the "official" representation of multiple context handlers is for Python 2 (as the ASTs there do not have a sequence of with_items). Turns out Python 2 just unfolds it into a series of nested with statements.

[tausbn]

No other objections, it seems, so I'll go ahead and merge this.

Thanks for the fix!