Treedb

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Add missing ``os.path.expanduser()`` to ``treedb.export()``.

``treedb.pd_read_sql()`` now uses the default query when called without
argument.

Add ``csv23`` as dependency (factored out stream handling).

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Rename ``treedb.create_engine()`` to ``treedb.set_engine()``
(backwards incompatible).

Improve ``treedb.backup()`` implementation.

Add helper functions and shortcuts.

Refactor ``subprocess`` usage.

Improve logging.

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Add ``treedb.backup()`` (requires Python 3.7+).

Add ``exclude_raw=False`` to ``treedb.export()``.

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Fix ``treedb.write_csv()`` endangerment_source column output.

Use ``os.path.expanduser()`` on filename arguments.

Fix setup.py old license classifier.

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Drop Python 2 and 3.5 support.

Parse endangerment source references into individual fields.

Switch license to MIT license.

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Make endangerment sources open-ended.