[Python] What does a bare asterisk do in a parameter list? What are "keyword-only" parameters?

What does a bare asterisk in the parameters of a function do?

When I looked at the pickle module, I see this:

pickle.dump(obj, file, protocol=None, *, fix_imports=True)


I know about a single and double asterisks preceding parameters (for variable number of parameters), but this precedes nothing. And I'm pretty sure this has nothing to do with pickle. That's probably just an example of this happening. I only learned its name when I sent this to the interpreter:

>>> def func(*):
... pass
...
File "<stdin>", line 1
SyntaxError: named arguments must follow bare *


If it matters, I'm on python 3.3.0.

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