Add warning for build_call_graph returning a set (#1396)

* Add warning for build_call_graph returning a set

- We changed build_call_graph to return a dictionary
as a part #845.
- This now emits a deprecation warning now that all the
built-in bloqs are converted.

* Add DeprecationWarning

---------

Co-authored-by: Matthew Harrigan <[email protected]>

qualtran/resource_counting/_call_graph.py

@@ -15,6 +15,7 @@
 """Functionality for the `Bloq.call_graph()` protocol."""
 
 import collections.abc
+import warnings
 from collections import defaultdict
 from typing import (
     Callable,
@@ -94,6 +95,11 @@ def _generalize_callees(
     callee_counts: Dict[Bloq, Union[int, sympy.Expr]] = defaultdict(lambda: 0)
     if isinstance(raw_callee_counts, set):
         raw_callee_iterator: Iterable[BloqCountT] = raw_callee_counts
+        warnings.warn(
+            "build_call_graph returning sets is deprecated (got {raw_callee_counts})."
+            "Please change build_call_graph for this bloq to use a dictionary.",
+            DeprecationWarning,
+        )
     else:
         raw_callee_iterator = raw_callee_counts.items()
     for callee, n in raw_callee_iterator: