Switch to explicit test for list/tuple

As per discussion in pypa#1769 (comment)

changelog.d/1769.change.rst

@@ -1,3 +1,3 @@
 The ``package_data`` and ``exclude_package_data`` arguments expect a dictionary whose values are a list of strings that represent paths/globs.
 Check these arguments more thoroughly, such that an error is raised when providing a dictionary with a string value.
-Also verify that dictionary values are multi-use iterables of strings (i.e. not single-use generators).
+Also verify that dictionary values are lists (or tuples) of strings (i.e. not single-use generators or sets).

setuptools/dist.py

@@ -214,9 +214,10 @@ def check_importable(dist, attr, value):
 def assert_string_list(dist, attr, value):
     """Verify that value is a string list"""
     try:
-        # verify that value is not a single-use iterable
-        assert list(value) == list(value)
-        # verify that iterable contains strings
+        # verify that value is a list or tuple to exclude unordered
+        # or single-use iterables
+        assert isinstance(value, (list, tuple))
+        # verify that elements of value are strings
         assert ''.join(value) != value
     except (TypeError, ValueError, AttributeError, AssertionError):
         raise DistutilsSetupError(