Ipyflow

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* Couple of minor bugfixes;

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* Relax gc thresholds to improve perf;

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* Improved support for Modin dataframes;

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* Better handling of namespace symbol contributions to slices;

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* Pin pyccolo to 0.0.39 exactly prevent forward compat issues;
* Fix a couple of hybrid-liveness-dag issues / corner cases;
* Stop reactive execution when exception is encountered;
* Use liveness_based exec_schedule for classic notebook frontend;

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* Use pyccolo >= 0.0.39 for better syntax augmentation;