Treedb

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Rename ``treedb.export()`` to ``treedb.csv_zipfile()``.

Update SQLAlchemy to 1.4 with ``FUTURE=True``:
- remove bound select (use sqlalchemy select with scalar,
iterrows, print_rows, write_csv, etc., or with connect)
- raw now records information about flag definitions
- improved ``raw.print_stats()`` order

Add ``print_versions()`` and ``print_dataset()``.

Simplify count queries in checks.

Improve package structure.

Improve block-style code-formatting.

Improve test environment: add ``--skip-slow`` and ``--log-sql``

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Add workaround for https://bugs.python.org/issue18199.

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Add support for new minimal countries format in glottolog ``md.ini`` files
(see https://github.com/glottolog/glottolog/pull/636). The previous full format
continues to be supported for reading older versions of the repository.

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Add Python 3.9 to test environments and tag support.

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Pin sqlalchemy dependency to version ``1.3.*``.

Add explicit ``.alias()`` or ``.as_scalar()`` to implicit subqueries.

Decrease progress ``logging`` verbosity.

Add test checksums for Glottolog ``v4.3-treedb-fixes``.

Improve test reporting, update test environment.

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Restrict foreign key pragma and regexp operator to ``sqlite3`` connections.