Fix equality under Python3.6.

Test failures revealed isinstance for types derived from tuple behaves
unexpectedly under Python3.6. Although this can be worked around by
saving off the mro in __init_subclass__ and using that to perform a
custom isinstance check, more investigation indicated two aspects of
the status quo were undesirable:

1. By returning NotImplemented we left equality up to foreign types
to decide.
2. The intended use of Collection subtypes is for the engine, which
treats all types as distinct; yet we would compare subtypes as equal.

As such implement exact type match based equality and document this.

src/python/pants/engine/collection.py

@@ -19,6 +19,8 @@ class Example:
 
         class Examples(Collection[Example]):
             pass
+
+    N.B: Collection instances are only considered equal if both their types and contents are equal.
     """
 
     @overload  # noqa: F811
@@ -36,9 +38,7 @@ def __getitem__(self, index: Union[int, slice]) -> Union[T, "Collection[T]"]:  #
         return self.__class__(cast(Tuple[T, ...], result))
 
     def __eq__(self, other: Any) -> bool:
-        if not isinstance(other, self.__class__):
-            return NotImplemented
-        return super().__eq__(other)
+        return type(self) == type(other) and super().__eq__(other)
 
     def __ne__(self, other: Any) -> bool:
         # We must explicitly override to provide the inverse of _our_ __eq__ and not get the

src/python/pants/engine/collection_test.py

@@ -46,6 +46,9 @@ def test_collection_reversed() -> None:
 
 
 def test_collection_equality() -> None:
+    assert () != Collection()
+    assert Collection() != ()
+
     assert Collection([]) == Collection([])
     c1 = Collection([1, 2, 3])
     assert c1 == Collection([1, 2, 3])