Fix PyMC3 after deprecation of step_kwargs and nuts_kwargs

The new release of PyMC3 (v3.9) has completely removed the nuts_kwargs and step_kwargs options, which breaks the test suite. This MR allows the PyMC3 implementation in bilby to work for version before and after 3.9.

Refs !811 (merged).

mentioned in merge request !811 (merged)

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• 4a2c812e - Switch to StrictVersion rather than packaging.version

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• 8ce9b4e4 - Move setting of default step_kwargs so that PyMC3 is not required when importing the class

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• bb27651c - Fix sampler test failure

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approved this merge request

merged

mentioned in commit 4893cd83

changed milestone to %1.0.0