Products.sqlalchemyda

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- fix test failures under Python 3.5 by pinning SQLAlchemy.
Pin can be dropped when support for Zope 4 is dropped.

- change package structure to move package code into a ``src`` subfolder

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- add linting configurations and apply results

- Package cleanup

- Add support for Python 3.5-3.9
(`8 <https://github.com/zopefoundation/Products.SQLAlchemyDA/pull/8>`_)

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URL fix in add form.

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Workarounds for edge case error conditions looking up or
creating underlying z3c.sqlalchemy ZopeWrapper instances.

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Beta Release: If you try out this version, please provide feedback!

Added public API for use within Zope acquisition context to access
the underlying zc3.sqlalchemy `ZopeWrapper` instance, with added
error handling and logging to deal with situations where Zope
context may have been lost.

Tests now assume a testrunner such as nose or py.test, and
testfixtures has been added as a testing dependency.

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Improved safety of non-acquisition public API by making registry no longer
contain Zope Persistent objects; instead storing and returning only the
underlying zc3.sqlalchemy `ZopeWrapper` instances. These objects
are plain Python objects in memory with no Persistent connection to ZODB.
(Thanks to Tres Seaver for the suggestion!)