Yatiml

first two fixes here add work-arounds that get us partially back up to YAML
1.2 and less surprising syntax rules.

* Switch back to YAML 1.2 floats
* Switch back to YAML 1.2 booleans
* Make enum values matching boolean constants work correctly

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Fixes
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The switch to PyYAML in the previous release also switched us back to YAML

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New functionality
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* Support for Python 3.12
* Improved error messages

Fixes
-----

* Now uses PyYAML rather than ruamel.yaml (fixes deprecation warnings)

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Incompatible changes
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* Ignore abstract base classes (abc.ABC and/or abstractmethod)

New functionality
-----------------

* Easier-to-understand error messages
* Installation via Conda (already worked, now documented)
* Small documentation improvements
* Python 3.11 support
* Compatibility with ruamel.yaml 0.17

Fixes
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* Bug in map_attribute_to_index

Removed
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* Support for Python 3.6

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New functionality
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* Support for Python 3.10 (worked fine already, now official)
* Error messages now quote attribute names, making them easier to read.


Fixes
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* `map_attribute_to_index()` caused load failure for dictionaries indexed by
a user-defined string class.
* All examples in the documentation now use the new `_yatiml_*` function names,
instead of the old ones without the leading underscore.


Removed
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* Loader and Dumper classes, which were deprecated and broken in 0.8.0. Please
use the new `load_function`, `dump_function` and `dumps_function` functions
instead.

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Incompatible changes
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* Accept explicit tags only if compatible with the recognised type(s)

New functionality
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* Support for untyped documents and attributes
* Support for Any-typed documents and attributes
* Support for Python dataclasses


Fixes
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* Dumping of OrderedDict to a file (but not to a string) produced a stray
!!omap.
* Various fixes and improvements to development infrastructure


Removed
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* Official support for Python 3.5, which is no longer supported upstream. It
will probably still work, but getting anything to install on 3.5 is getting to
be pretty difficult so it's probably time to upgrade.