Xyzservices

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Providers:

- Added ``OpenStreetMap.BlackAndWhite`` (83)
- Added ``Gaode`` tiles (``Normal`` and ``Satellite``) (83)
- Expanded ``NASAGIBS`` tiles with ``ModisTerraBands721CR``, ``ModisAquaTrueColorCR``, ``ModisAquaBands721CR`` and ``ViirsTrueColorCR`` (83)
- Added metadata to ``Strava`` maps (currently down) (83)

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New functionality:

- Added ``TileProvider.from_qms()`` allowing to create a ``TileProvider`` object from the remote [Quick Map Services](https://qms.nextgis.com/about) repository (#71)
- Added support of ``html_attribution`` to have live links in attributions in HTML-based outputs like leaflet (60)
- New ``Bunch.flatten`` method creating a flat dictionary of ``TileProvider`` objects based on a nested ``Bunch`` (68)
- Added ``fill_subdomain`` keyword to ``TileProvider.build_url`` to control ``{s}`` placeholder in the URL (75)
- New Bunch.filter method to filter specific providers based on keywords and other criteria (76)

Minor enhancements:

- Indent providers JSON file for better readability (64)
- Support dark themes in HTML repr (70)
- Mark broken providers with ``status="broken"`` attribute (78)
- Document providers requiring registrations (79)

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The initial release provides ``TileProvider`` and ``Bunch`` classes and an initial set of providers.