Products.zsqlmethods

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- Add flake8 code checking.

- Add test coverage computation.

- Various small Python 3 compatibility changes.

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- Fix syntax error in `Shared/DC/ZRDB/dbi_db.py`.

- More PEP8 compliance.

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- Added tox configuration.

- Python 3 compatibility.

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- Target use with Zope 4: no longer support 2.13.x.

- Make webdav from ZServer optional.
[dataflake]

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- Strip ``NUL`` bytes when quoting SQL string literals.

- Fixed a bug which might occur on Windows when two cache entries have been
stored without `time.time()` having changed and the cache gets purged
afterwards.

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- Copy code from `App.Extensions` to keep compatibility with Zope 2.14.