Support GDAL 3.7 & 3.8

[davemfish]

As far as I can see, there are no breaking changes to worry about. But there are some FutureWarnings starting in 3.7 that we tell pytest to elevate to errors, so our test suite fails for this reason. For example,

=================================== ERRORS ====================================
_____________________ ERROR collecting tests/test_hra.py ______________________
tests\test_hra.py:22: in <module>
    _SRS = osr.SpatialReference()
C:\Users\runneradmin\micromamba\envs\env\Lib\site-packages\osgeo\osr.py:1027: in __init__
    _WarnIfUserHasNotSpecifiedIfUsingExceptions()
C:\Users\runneradmin\micromamba\envs\env\Lib\site-packages\osgeo\osr.py:410: in _WarnIfUserHasNotSpecifiedIfUsingExceptions
    warnings.warn(
E   FutureWarning: Neither osr.UseExceptions() nor osr.DontUseExceptions() has been explicitly called. In GDAL 4.0, exceptions will be enabled by default.

[emlys]

This was addressed in #1546