Mercantile

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- Change a list comprehension to a generator expression in simplify().
- Change DeprecationWarning introduced in 1.1.3 to a UserWarning to increase
visibility.
- Ensure symmetric InvalidLatitudeErrors at both poles (106).

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- Warn about deprecation of support when mercantile is imported with Python
versions < 3. Mercantile 2.0 will not be compatible with Python 2.7.
- The bounding tile of the bounds of a tile is now that same tile (100).

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- fid of 0 is now allowed by the ``feature()`` function (85).
- Passing the bounds of a tile to ``tiles()`` now yields exactly that tile
only (given the correct zoom), resolving 84 and 87.
- QuadKeyError derives from ValueError again, resolving issue 98, but only
until version 2.0. A deprecation warning explaining this is raised from
quadkey_to_tile just before QuadKeyError is raised.
- Format source using black.

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- Update tests to work with pytest 5.

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- A zoom keyword argument has been added to both ``children()`` and
``parent()``, allowing the user to specify the zoom level for each (94).
- A new ``simplify()`` function merges child to parent tiles, upwards,
producing the shortest sequence of tiles that cover the same space (94).
- The mercantile module now raises only exceptions deriving from
MercantileError. Such errors indicate improper usage of the mercantile
module. The occurance of a builtin exception indicates a bug in mercantile.

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- Added missing docstrings (80).