Nano-utils

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* Added a decorator for constructing positional-only signatures.
* Added two mappings for creating structured dtypes.
* Added two mapping baseclasses for user defined mappings.

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* Reimplement ``RecursiveValuesView.__eq__`` in terms of ``collections.Counter`` comparisons.

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* Added the ``fullmatch`` argument to ``VersionInfo.from_str``.
* The h5py-related ``recursive_keys``, ``recursive_values`` and ``recursive_items``
functions now return ``collections.abc.MappingView`` instances, rather than generators.
These mappingviews are available via the ``RecursiveKeysView``,
``RecursiveValuesView`` and ``RecursiveItemsView`` classes.

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* Added ``LazyImporter`` and ``MutableLazyImporter``, two classes for lazily importing objects.

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* Allow ``SequenceView.index`` and ``SequenceView.count`` to pass on arbitrary parameters.
* Reworked ``SequenceView.__repr__`` to use ``pprint``.

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* Added the new ``SequenceView`` and ``CatchErrors`` classes.
* Drop support for Python 3.6.