Geoalchemy2

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* Improve doc formatting adrien-berchet (369)

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* Add Alembic helpers for Add/Drop spatial columns with SQLite adrien-berchet (362)
* Add GitHub URL for PyPi andriyor (364)
* Add a simple API to register custom mapping for SQLite function names adrien-berchet (360)
* Example of specific compilation for sided buffer with SQLite dialect adrien-berchet (361)
* Remove metaclass as in SQLAlchemy, rework the tests and some reformatting adrien-berchet (357)
* Use RecoverGeometryColumn instead of AddGeometryColumn with SQLite adrien-berchet (354)
* Fix spatial index declaration adrien-berchet (344)

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* Use 'load_dialect_impl()' instead of 'impl' to get the type of the TypeDecorator objects adrien-berchet (343)

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* Fix creation of columns using a TypeDecorator adrien-berchet (343)

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* Fix cache warnings adrien-berchet (338)
* Drop support for Python < 3.6 adrien-berchet (337)
* Change KeyError into AttributeError in the comparator_factory of CompositeType adrien-berchet (335)
* Add SummaryStats type for the ST_SummaryStatsAgg function adrien-berchet (334)

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* Fix warnings in preparation for SQLAlchemy v2.0 robintw (331)