Edtf

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* Breaking Changes: Implementation of the newer specifications from `https://www.loc.gov/standards/datetime/`::

Differences
This specification differs from the earlier draft as follows:

- the unspecified date character (formerly lower case ‘u’) is superseded by the character (upper case) 'X';
- Masked precision is eliminated;
- the uncertain and approximate qualifiers, '?' and '~', when applied together, are combined into a single qualifier character '%';
- “qualification from the left” is introduced and replaces the grouping mechanism using parentheses;
- the extended interval syntax keywords 'unknown' and 'open' have been replaced with null and the double-dot notation ['..'] respectively;
- the year prefix 'y' and the exponential indicator 'e', both previously lowercase, are now 'Y' and 'E' (uppercase); and
- the significant digit indicator 'p' is now 'S' (uppercase).

* Renaming of the project to edtf2: As this project seems to have no longer support from the creator `The Interaction Consortium` we decided to fork it and release it under a new name by our own
* Author: https://github.com/muellersSAW

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* Remove 1 AD - 9999 AD restriction on date ranges imposed by Python's
``datetime`` module (26).

**WARNING**: This involves a breaking API change where the following methods
return a ``time.struct_time`` object instead of ``datetime.date`` or
``datetime.datetime`` objects::

lower_strict()
upper_strict()
lower_fuzzy()
upper_fuzzy()

* Add `jdutil` library code by Matt Davis at
`https://gist.github.com/jiffyclub/1294443`_ to convert dates to numerical
float representations.

* Update `EDTFField` to store derived upper/lower strict/fuzzy date values as
numerical values to Django's `FloatField` fields, when available, to permit
storage of arbitrary date/time values.

The older approach where `DateField` fields are used instead is still
supported but not recommended, since this usage will break for date/time
values outside the range 1 AD to 9999 AD.

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* Python 3 compatibility.
PR: https://github.com/ixc/python-edtf/pull/25
Author: https://github.com/ahankinson

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* Fix lower/upper fuzzy values for outlier date values

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* EDTF fields are now cleared when their source fields go.
* Improved clarity of parser errors.
* Improved parser accuracy and resilience (20, 22).

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* Order-of-magnitude performance improvement in parsing.
* EDTFFields are now serialised EDTF objects, rather than unparsed EDTF text
strings.