`UPath.rename` should raise on relative paths if no `.cwd()` is implemented

[ap--]

This is currently incorrectly implemented and incorrectly checked in the tests.

universal_pathlib/upath/tests/cases.py

Lines 290 to 302 in c31e910

	def test_rename2(self):
	upath = self.path.joinpath("folder1/file2.txt")
	target = "file2_renamed.txt"
	moved = upath.rename(target)
	target_path = upath.parent.joinpath(target).resolve()
	assert target_path == moved
	assert not upath.exists()
	assert moved.exists()
	# reverse with a relative path as UPath
	back = moved.rename(UPath("file2.txt"))
	assert back == upath
	assert not moved.exists()
	assert back.exists()

universal_pathlib/upath/core.py

Line 1792 in c31e910

def rename(