Fix cupy fft __all__

Author: asmeurer

array_api_compat/cupy/fft.py

@@ -1,5 +1,12 @@
 from cupy.fft import * # noqa: F403
-from cupy.fft import __all__ as fft_all
+# cupy.fft doesn't have __all__. If it is added, replace this with
+#
+# from cupy.fft import __all__ as linalg_all
+_n = {}
+exec('from cupy.fft import *', _n)
+del _n['__builtins__']
+fft_all = list(_n)
+del _n
 
 from ..common import _fft
 from .._internal import get_xp