Let user decide which cupy version to use (#67)

* Let user decide which cupy version to use

Remove cupy from dependencies and add three extras where each requires a different cupy version: cupy (building from source), and the two prebuild cupy-cuda12x and cupy-cuda11x

* Fix ruff B028 and E501

* Remove cupy install option via extras

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Wei Ji <[email protected]>

README.md

@@ -16,6 +16,8 @@ Interface for using cupy in xarray, providing convenience accessors.
 
 ## Installation
 
+> `cupy-xarray` will use an existing cupy installation, hence cupy needs to be installed manually! Please follow cupy's install instructions at <https://docs.cupy.dev/en/stable/install.html>.
+
 From anaconda:
 
 ```console

cupy_xarray/accessors.py

@@ -1,6 +1,16 @@
+import warnings
 from typing import TYPE_CHECKING, Any
 
-import cupy as cp
+try:
+    import cupy as cp
+except ImportError as e:
+    warnings.warn(
+        "Cupy is not installed. cupy-xarray expects cupy to be manually installed. Please install "
+        "cupy by following the instructions at https://docs.cupy.dev/en/stable/install.html.",
+        ImportWarning,
+        stacklevel=2,
+    )
+    raise e
 from xarray import (
     DataArray,
     Dataset,

docs/index.md

@@ -16,6 +16,8 @@ CuPy-Xarray is a Python library that leverages [CuPy](https://cupy.dev/), a GPU
 
 ## Installation
 
+> `cupy-xarray` will use an existing cupy installation, hence cupy needs to be installed manually! Please follow cupy's install instructions at <https://docs.cupy.dev/en/stable/install.html>.
+
 CuPy-Xarray can be installed using `pip` or `conda`:
 
 From Conda Forge:

pyproject.toml

@@ -15,7 +15,6 @@ classifiers = [
 ]
 dynamic = ["version"]
 dependencies = [
-    "cupy",
     "xarray>=2024.02.0",
 ]