Consider removing six, past, and future support #13567

Since python2 is now no longer supported, I think it might be a good time to remove special casing of these libraries.

Why?

1. None of them are required for python3+ code
2. All their features are now (long) supported by python3+
3. Their support is clearly a hack
4. Code to support them is quite complex:

   mypy/mypy/semanal.py

   Lines 1971 to 2018 in 840a310

   	def update_metaclass(self, defn: ClassDef) -> None:
   	"""Lookup for special metaclass declarations, and update defn fields accordingly.
   	* six.with_metaclass(M, B1, B2, ...)
   	* @six.add_metaclass(M)
   	* future.utils.with_metaclass(M, B1, B2, ...)
   	* past.utils.with_metaclass(M, B1, B2, ...)
   	"""
   	# Look for six.with_metaclass(M, B1, B2, ...)
   	with_meta_expr: Expression | None = None
   	if len(defn.base_type_exprs) == 1:
   	base_expr = defn.base_type_exprs[0]
   	if isinstance(base_expr, CallExpr) and isinstance(base_expr.callee, RefExpr):
   	base_expr.accept(self)
   	if (
   	base_expr.callee.fullname
   	in {
   	"six.with_metaclass",
   	"future.utils.with_metaclass",
   	"past.utils.with_metaclass",
   	}
   	and len(base_expr.args) >= 1
   	and all(kind == ARG_POS for kind in base_expr.arg_kinds)
   	):
   	with_meta_expr = base_expr.args[0]
   	defn.base_type_exprs = base_expr.args[1:]
   	# Look for @six.add_metaclass(M)
   	add_meta_expr: Expression | None = None
   	for dec_expr in defn.decorators:
   	if isinstance(dec_expr, CallExpr) and isinstance(dec_expr.callee, RefExpr):
   	dec_expr.callee.accept(self)
   	if (
   	dec_expr.callee.fullname == "six.add_metaclass"
   	and len(dec_expr.args) == 1
   	and dec_expr.arg_kinds[0] == ARG_POS
   	):
   	add_meta_expr = dec_expr.args[0]
   	break
   	metas = {defn.metaclass, with_meta_expr, add_meta_expr} - {None}
   	if len(metas) == 0:
   	return
   	if len(metas) > 1:
   	self.fail("Multiple metaclass definitions", defn)
   	return
   	defn.metaclass = metas.pop()

5. There are multiple places where six. / etc special cases are used: mypy/stubgen.py, mypy/semanal.py, mypyc/irbuild/for_helpers.py

I don't see much benefit in keeping them.

Related #12237