Rasterio

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- Fix for an unchecked NULL pointer introduced in 1.0.19 that could result in
segmentation fault on import of rasterio._env on Linux when GDAL data files
are not available (for example, with wheels as on PyPI).

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- Do not set GDAL_DATA and PROJ_LIB when data files can be found at their built
in locations (1631).
- Add linear_units property to CRS (1638).
- Ensure that AWS_NO_SIGN_REQUESTS is sufficient for accessing public S3
datasets and that import of boto3 is not required (1637).
- An out_dtype parameter has been added to DatasetReaderBase.read, enabling
on-the-fly casting of raster values to the desired type (1637). This uses
the same latent feature used to get support for "int8" data.
- Restore pickle protocol for CRS, using WKT as state (1625).
- Support for signed 8-bit integer datasets ("int8" dtype) has been added
(1595).

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- Fix a regression reported in 1623.

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- Fix a regression in evaluation of CRS equality (1620).

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- A bug preventing GCPs from being created with new BufferedDatasetWriter
instances (1600) has been fixed (1610).
- A previously unreported bug preventing BufferedDatasetWriters from being
opened in r+ mode has been fixed.
- A regression in creating CRS objects from PROJ4 strings that include
"+wktext" (1609) has been fixed.
- Regressions in str representations of empty CRS objects and the handling of
unreferenced datasets in rasterio._base have been fixed (1616).
- GDAL seems to work best if GDAL_DATA is set as early as possible. Ideally it
is set when building the library or in the environment before importing
Rasterio, but for wheels we patch GDAL_DATA into os.environ when rasterio.env
is imported. This resolves 1611.

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- Google cloud storage support was *not* in fact added in 1.0.14, but is
present in 1.0.15.