Drop support for Python 3.7

[srittau]

Part of #10113

[srittau]

This PR contains only infrastructure work. Changes to the stubs themselves will follow in a subsequent PR.

[AlexWaygood]

You can also delete the lint in check_new_syntax.py that checks we're not using any PEP-570 syntax (but it's also fine to leave that to a followup PR):

typeshed/tests/check_new_syntax.py

Lines 29 to 49 in 54447f6

	class PEP570Finder(ast.NodeVisitor):
	def __init__(self) -> None:
	self.lineno: int | None = None
	def _visit_function(self, node: ast.FunctionDef | ast.AsyncFunctionDef) -> None:
	old_lineno = self.lineno
	self.lineno = node.lineno
	self.generic_visit(node)
	self.lineno = old_lineno
	visit_FunctionDef = visit_AsyncFunctionDef = _visit_function
	def visit_arguments(self, node: ast.arguments) -> None:
	if node.posonlyargs:
	assert isinstance(self.lineno, int)
	errors.append(
	f"{path}:{self.lineno}: PEP-570 syntax cannot be used in typeshed yet. "
	f"Prefix parameter names with `__` to indicate positional-only parameters"
	)
	self.generic_visit(node)

[AlexWaygood]

And this can also be deleted:

typeshed/CONTRIBUTING.md

Lines 282 to 286 in 54447f6

	Accepted features that *cannot* yet be used in typeshed include:
	- [PEP 570](https://www.python.org/dev/peps/pep-0570/) (positional-only
	arguments): see [#4972](https://github.com/python/typeshed/issues/4972),
	use argument names prefixed with `__` instead

[srittau]

And this can also be deleted:

typeshed/CONTRIBUTING.md

Lines 282 to 286 in 54447f6

	Accepted features that *cannot* yet be used in typeshed include:
	- [PEP 570](https://www.python.org/dev/peps/pep-0570/) (positional-only
	arguments): see [#4972](https://github.com/python/typeshed/issues/4972),
	use argument names prefixed with `__` instead

I removed it, but didn't add it to supported features, as it's not really a typing feature.

[AlexWaygood]

Thanks!