Pygeoogc

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- Re-wrote ``wms_bybox`` function as a class called ``WMS`` with a similar
interface to the ``WFS`` class.
- Added support for WMS 1.3.0 and WFS 2.0.0.
- Added a custom ``Exception`` for the threading function called ``ThreadingException``.
- Add ``always_xy`` flag to ``WMS`` and ``WFS`` which is False by default. It is useful
for cases where a web service doesn't change the axis order from the transitional
``xy`` to ``yx`` for versions higher than 1.3.0.

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- Remove unnecessary transformation of the input bbox in WFS.
- Use ``setuptools_scm`` for versioning.

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- Add the missing ``max_pixel`` argument to the ``wms_bybox`` function.
- Change the ``onlyIPv4`` method of ``RetrySession`` class to ``onlyipv4``
to conform to the ``snake_case`` convention.
- Improve docstrings.

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- Initial release.