Harmonizing the JSON serialization of non-finite float values with browsers and other languages

[encukou (Petr Viktorin)]

I wouldn’t be surprised if someone out there was using allow_nan=None to mean allow_nan=False. We shouldn’t change the meaning silently.
Solutions I can think of:

• Adding a new argument, like nonfinite=None. Initially it would only take None but it could be extended in the future, if necessary.

• Using a specific value that’s very unlikely to be used currently, like allow_nan='null'. To allow future extensions, start emitting a DeprecationWarning when a non-bool is passed to allow_nan.

  This would technically be a backwards incompatible change, but it could get a SC exception.