Improve distutils.dist.Distribution from typeshed's setuptools.setup

[Avasam]

Taken from https://github.com/python/typeshed/blob/9b7b2bafab7599195f400e1bf9f31620321aeb2e/stubs/setuptools/setuptools/__init__.pyi#L61-L105

These are used for pypa/setuptools#5021

[Avasam]

typeshed typed this as Sequence, but as far as I can see

distutils/distutils/command/build_ext.py

Line 178 in 603b94e

self.extensions = self.distribution.ext_modules

and

distutils/distutils/command/build_ext.py

Lines 370 to 383 in 603b94e

	def check_extensions_list(self, extensions) -> None: # noqa: C901
	"""Ensure that the list of extensions (presumably provided as a
	command option 'extensions') is valid, i.e. it is a list of
	Extension objects. We also support the old-style list of 2-tuples,
	where the tuples are (ext_name, build_info), which are converted to
	Extension instances here.
	Raise DistutilsSetupError if the structure is invalid anywhere;
	just returns otherwise.
	"""
	if not isinstance(extensions, list):
	raise DistutilsSetupError(
	"'ext_modules' option must be a list of Extension instances"
	)

means it has to be a list.