Resolve new flake8 errors thanks to recent flake8-bugbear feature releases

[ichard26]

I recently accidentally cleared out all of my pre-commit hook installations and discovered using the latest version of everything nets us some new warnings:

❯ pre-commit run -a
black....................................................................Passed
Check pre-commit rev in example..........................................Passed
Check black version in the basics example................................Passed
flake8...................................................................Failed
- hook id: flake8
- exit code: 1

src/black/linegen.py:668:13: B020 Found for loop that reassigns the iterable it is iterating with each iterable value.
src/black/parsing.py:228:25: B020 Found for loop that reassigns the iterable it is iterating with each iterable value.

mypy.....................................................................Passed
prettier.................................................................Passed
Fix End of Files.........................................................Passed
Trim Trailing Whitespace.................................................Passed

black/src/black/parsing.py

Lines 219 to 229 in 3800ebd

	if isinstance(value, list):
	for item in value:
	# Ignore nested tuples within del statements, because we may insert
	# parentheses and they change the AST.
	if (
	field == "targets"
	and isinstance(node, (ast.Delete, ast3.Delete))
	and isinstance(item, (ast.Tuple, ast3.Tuple))
	):
	for item in item.elts:
	yield from stringify_ast(item, depth + 2)

black/src/black/linegen.py

Lines 665 to 677 in 3800ebd

	def dont_increase_indentation(split_func: Transformer) -> Transformer:
	"""Normalize prefix of the first leaf in every line returned by `split_func`.
	This is a decorator over relevant split functions.
	"""
	@wraps(split_func)
	def split_wrapper(line: Line, features: Collection[Feature] = ()) -> Iterator[Line]:
	for line in split_func(line, features):
	normalize_prefix(line.leaves[0], inside_brackets=True)
	yield line
	return split_wrapper

[ichard26]

Also visible here when the cache finally got evicted / expired: https://github.com/psf/black/runs/5668882528?check_suite_focus=true