With SQLAlchemy warnings enabled, SQLAlchemy 1.4 gives the following warning.

MovedIn20Warning: The ``declarative_base()`` function is now available as sqlalchemy.orm.declarative_base(). (deprecated since: 1.4) (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
    Base = declarative_base()

The rationale of the move is explained here: https://docs.sqlalchemy.org/en/14/changelog/migration_20.html#declarative-becomes-a-first-class-api

mbdata's requirements.txt does not specify any fixed SQLAlchemy version
so we need to account for versions before 1.4 as well where this import
does not exist.

In Python 3, the data model was changed to not provide a __hash__ method
if __eq__ is overriden in a class. PartialDate defines __eq__ but not
__hash__ and is thus unhashable. To quote the docs,

> A class that overrides __eq__() and does not define __hash__() will have
its __hash__() implicitly set to None. When the __hash__() method of a
class is None, instances of the class will raise an appropriate TypeError
when a program attempts to retrieve their hash value, and will also be
correctly identified as unhashable when checking isinstance(obj, collections.abc.Hashable).

(from https://docs.python.org/3.7/reference/datamodel.html#object.__hash__)

We store PartialDate's in a set in [SIR](https://github.com/metabrainz/sir) so it would be nice to make it hashable.