Make cassettes thread-safe

[sigmavirus24]

I'm wondering if there's a need to test items using requests in thread-safe manner and how that would work with betamax. Currently the access to cassette files is not threadsafe and I'm wondering what would be necessary to ensure thread-safety.

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[sigmavirus24]

So, we'd probably have to use pylockfile to ensure we don't update the same cassette twice. That would probably give us 90% of what we need. Namely, this would be great for the case where we are testing an application that's sharing a session between threads. Even though this isn't a recommendation requests makes, Betamax should still attempt to support it.

Further, I think the dependency on pylockfile should be optional that could be installed from an extra for betamax, e.g., pip install betamax[threadsafe].

The code to integrate this with the current Cassette class should be minimal.