Dbfpy3

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Fixes:
- Fix encode in DbfMemoField.
- Fix new MemoFile header.
- Fix tests.

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Released at PyPi.

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Fix Y2K issue with Last Update field in the header (sf bug 3065838).

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Ignore leading and trailing zero bytes in numeric fields.

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Support for writing empty date values.

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Numeric decoder returns float when the value contains decimal point
without regard to the number of decimal places declared in the header.