Katdal

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* Switch to PyJWT 2 and Python 3.6, cleaning up Python 2 relics (321 - 323)
* Allow preselection of channels and dumps upon katdal.open() to save time and memory (324)
* Allow user to select fields, scans and antennas in mvftoms (269)
* Support h5py 3.0 string handling in MVF3 (331)
* Refactor requirement files to remove recursive dependencies (329)

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* This is the last release that will support Python 3.5
* Pin PyJWT version to 1.x to avoid breaking API changes (320)
* Van Vleck correction! (autocorrelations only, though) (316)
* Expose excision, aka raw weights (308)
* Better unit testing of DataSource and S3ChunkStore in general (319)
* Support indexed telstate keys (the 1000th cut that killed Python 2) (304)
* Split out separate utility classes for Minio (310)
* Fix filtering of sensor events with invalid status (306)

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* This is the last release that will support Python 2 (python2 maintenance branch)
* New 'time_offset' sensor property that adjusts timestamps of any sensor (307)
* Fix calculation of cbf_dump_period for 'wide' / 'narrowN' instruments (301)
* Increase katstore search window by 600 seconds to find infrequent updates (302)
* Refactor SensorData to become a lazy abstract interface without caching (292)
* Refactor SensorCache to use MutableMapping (300)
* Fix rx_serial sensor use and file mode warning in MVFv3 files (298, 299)

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* Improve S3 chunk store: check tokens, improve timeouts and retries (272 - 277)
* Retry truncated reads and 50x errors due to S3 server overload (274)
* Apply flux calibration if available (278, 279)
* Improve mvf_rechunk and mvf_read_benchmark scripts (280, 281, 284)
* Fix selection by target description (271)
* Mark Python 2 support as deprecated (282)

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* Make L2 product by applying self-calibration corrections (253 - 256)
* Speed up uvw calculations (252, 262)
* Produce documentation on readthedocs.org (244, 245, 247, 250, 261)
* Clean up mvftoms and fix REST_FREQUENCY in SOURCE sub-table (258)
* Support katstore64 API (265)
* Improve chunk store: detect short reads, speed up handling of lost data (259, 260)
* Use katpoint 0.9 and dask 1.2.1 features (262, 243)

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* Load RDB files straight from archive (233, 241)
* Retrieve raw sensor data from CAM katstore (234)
* Work around one-CBF-dump offset issue (238)
* Improved MS output: fixed RECEPTOR_ANGLE (230), added WEIGHT_SPECTRUM (231)
* Various optimisations to applycal (224), weights (226), S3 reads (229)
* Use katsdptelstate 0.8 and dask 1.1 features (228, 233, 240)