updated requirements for py39 (#588)

* tightened xarray requirements for py39 to avoid resampling error with pandas<2.0.0

* improved ordering and explanation in comments

setup.cfg

@@ -34,8 +34,8 @@ install_requires =
  #contour colorbar on uniform values fails with matplotlib<3.7.0, several other features with matplotlib<3.6.0
  matplotlib>=3.7.0
  #pandas<2.0.0 in case of py38 to avoid conflict with xarray<2023.3.0: https://github.com/Deltares/xugrid/issues/78#issuecomment-1597723955 (xarray 2023.1.0 is the latest py38 release, and py38 is still supported by dfm_tools)
+ #pandas>=1.4 is required by xarray>=2023.3.0
  pandas<2.0.0;python_version<='3.8'
- #pandas>=1.4 is required by xarray>=2023.3.0. TODO: if pandas>=2.0.0 is installed, xarray>=2023.3.0 is required (not supported by py38)
  pandas>=1.4.0
  #shapely<2.0.0 give "AttributeError: module 'shapely' has no attribute 'GeometryType'"
  shapely>=2.0.0
@@ -45,9 +45,12 @@ install_requires =
  fiona>=1.9
  #contextily<1.0.0 is from April 2020
  contextily>=1.0.0
+ #xarray<2023.4.0 conflicts with pandas<2.0.0 for resampling, only available for py39
  #xarray<2022.6.0 did not find certain variable in dataset in test_data_map testcase. TODO: lower xarray/dask to 2022.6.0 when xugrid>0.6.5 is available
+ xarray>=2023.4.0;python_version>='3.9'
  xarray>=2022.11.0
- #aligned with xarray, no particular reason
+ #dask is aligned with xarray, no particular reason
+ dask>=2023.4.0;python_version>='3.9'
  dask>=2022.11.0
  #netcdf4<1.5.4 pip install fails in py39
  netcdf4>=1.5.4

tests/test_external_packages.py

@@ -107,9 +107,11 @@ def test_xugrid_opendataset_ugridplot_contour_with_colorbar():
 def test_xarray_pandas_resample():
  """
  this one fails with xarray<2023.3.0 (required for test_opendataset_ugridplot()) and pandas>1.5.3: https://github.com/Deltares/xugrid/issues/78#issuecomment-1597723955
- pandas 1.5.3 works with xarray<2023.3.0
- pandas 2.0.* fails with xarray<2023.3.0
- pandas 2.0.* works with xarray==2023.5.0
+ pandas<1.5.3 works with xarray (any version)
+ pandas>=2.0.0 fails with xarray<2023.3.0
+ pandas>=2.0.0 cannot be installed with xarray==2023.3.0 (latter requires pandas<2 and >=1.4)
+ pandas>=2.0.0 works with xarray>=2023.4.0
+ therefore, xarray>=2023.4.0 works pandas (any version)
  """
  ds = xr.tutorial.load_dataset("air_temperature")
  ds.resample(time='D')