chore: Remove numpy pin, pin datashader, drop Python 3.7

[jthetzel]

Proposed Changes

• Pin datashader version to 0.15.0 and remove numpy version pin.
• Require Python >=3.8.

Xarray-spatial tests were failing due to datashader calling numpy.warnings, which is removed in numpy >=1.24.0 ( #779 ). Datashader 0.15.0 removes calls to numpy.warnings (pr: holoviz/datashader#1176 , changelog: https://github.com/holoviz/datashader/releases/tag/v0.15.0). This pull requests removes the numpy <= 1.23.4 pin in favour of a datashader >= 1.15.0 pin.

Datashader requires Python >=3.8, so this also drops support for Python 3.7.

[jthetzel]

Datashader 0.15.0 supports Python >= 3.8:
https://pypi.org/project/datashader/0.15.0/
(thus tests are failing for Python 3.7)

[jthetzel]

#770 considers dropping Python 3.7 support upon addition of Python 3.11 support. I'll defer to the maintainers.

[jthetzel]

To accomodate Datashader 0.15.0, this pull request also drops Python 3.7 support.

[brendancol]

@jthetzel thanks so much for doing this. @thuydotm will check and merge

[thuydotm]

Thanks @jthetzel! The new changes look good to me, merging.