Posting this here instead of Bitbucket because I'm really unsure about whether or not this is user error. Something simple like this broke after upgrading to the latest version:
>>> from ruamel import yaml
>>> test_str = '{"in":{},"out":{}}'
>>> yaml.safe_load(test_str)
ruamel.yaml.parser.ParserError: while parsing a flow mapping
in "<byte string>", line 1, column 1:
{"in":{},"out":{}}
^ (line: 1)
expected ',' or '}', but got '<scalar>'
in "<byte string>", line 1, column 6:
{"in":{},"out":{}}
^ (line: 1)
Putting spaces after "in" and "out" resolves the issue.