Fix pushsource-ls unable to serialize frozendict (on some versions)

On at least some pyyaml and frozendict versions, it is necessary to
explicitly add a representer for the frozendict type, as it will not
automatically be processed in the same way as a regular dict.

src/pushsource/_impl/list_cmd.py

@@ -29,6 +29,7 @@
 
 import attr
 import yaml
+from frozendict.core import frozendict
 
 import pushsource
 from pushsource import Source
@@ -57,6 +58,7 @@ def add_enum_representers(cls):
 
 
 ItemDumper.add_enum_representers()
+ItemDumper.add_representer(frozendict, ItemDumper.represent_dict)
 
 
 def format_python(item):