BLD: Use relative path in setup.py for pxd includes (#20924)

Author: TomAugspurger

setup.py

@@ -450,7 +450,7 @@ def srcpath(name=None, suffix='.pyx', subdir='src'):
 
 
 def pxd(name):
-    return os.path.abspath(pjoin('pandas', name + '.pxd'))
+    return pjoin('pandas', name + '.pxd')
 
 
 # args to ignore warnings