Concatenate to support Ellipsis argument

[Daraan]

Fixes #110.

This PR adds support for Ellipsis in Concatenate for the Python 3.8-3.10 backports.

Does not fix:

• Concatenate implementation generates runtime exception if parameterized by ... #48 : Using Ellipsis as argument for Callable with a generic..
  • <= 3.9: because of typing.Callable limitations
  • For 3.10 needs modification of _ConcatenateGenericAlias

Changes:
- Changed _concatenate_getitem to reflect the Python3.11 typing module that allows the Ellipsis parameter.
- For Python 3.10 uses typing._ConcatenateGenericAlias and _concatenate_getitem
- For <=Python 3.9 the typing_extensions._ConcatenateGenericAlias and _concatenate_getitem like before
- For <3.9.2 uses a Dummy class that is temporarily swapped with an Ellipsis input
- added tests and updated tests

[Daraan]

I am sorry I had to recreate #442.

Latest change since then:

-        if not TYPING_3_11_0:
-            with self.assertRaisesRegex(
-                TypeError,
-                'each arg must be a type',
-           ):
-              Concatenate[1, P]
# different location
+    def test_invalid_use(self):
+       # Assure that `_type_check` is called.
+        P = ParamSpec('P')
+        with self.assertRaisesRegex(
+            TypeError,
+            "each arg must be a type",
+        ):
+            Concatenate[(str,), P]