MAINT: inline the check for complex dtypes

Preferred as long as xp.isdtype is not universally available
(looking at you pytorch)

Author: ev-br

array_api_tests/test_linalg.py

@@ -907,8 +907,8 @@ def true_trace(x_stack, offset=0):
 
 
 def _conj(x):
-    """Work around xp.conj rejecting floats."""
-    if xp.isdtype(x.dtype, 'complex floating'):
+    # XXX: replace with xp.dtype when all array libraries implement it 
+    if x.dtype in (xp.complex64, xp.complex128):
         return xp.conj(x)
     else:
         return x