Use `skipkeys` when serializing payload

[lbolla]

See comment #1155 (comment)

When serializing error data (traceback, local vars, etc.) to be sent to the backend, json.dumps is used, in raven.utils.json. json.dumps can fail, e.g. when local data has the form:

{ (): "foo" }

which cannot be serialized to json (because of the tuple key).
By using json.dumps(skipkeys=True) these kind of keys will be ignored.

[ohimors]

@lbolla When is this going to be done? Isn't this a one-line fix? Does this product even work?

File "/usr/local/lib/python3.6/site-packages/raven/utils/json.py", line 63, in dumps\n    return json.dumps(value, cls=BetterJSONEncoder, **kwargs)\n  File "/usr/local/lib/python3.6/json/__init__.py", line 238, in dumps\n    **kw).encode(obj)\nTypeError: __init__() got an unexpected keyword argument \'encoding\'\n'

[lbolla]

@ohimors I've already submitted a PR ages ago: #1350
But I've since moved to use the new "Sentry SDK", given that "raven-python" is deprecated.
I don't know if the maintainers are actively working on Raven anymore.

[untitaker]

I'll look into this on Monday. We still want to fix bugs in Raven.

[untitaker]

• The encoding error only happens when another exception is already being thrown on Python 2. So the real bug here is that the exception for e.g. ()-keys is clouded by this error, but it's not inherently broken under Python 2.
• We cannot use skip_keys. It's way too broad in how it operates and discarding is not the way to go.
• We already stringify dictionary keys in raven.serializers.base.DictSerializer. I wonder why this is not applied in this case.