Change the namespace convention to xp (borrowed from CuPy)

Author: rgommers

spec/purpose_and_scope.md

@@ -47,11 +47,9 @@ To address both issues, a uniform method must be provided by a conforming
 implementation to access the API namespace, namely a function:
 
 ```
-mod = x.__array_namespace__(api_version='2020.10')
+xp = x.__array_namespace__(api_version='2020.10')
 ```
 
-TBD: what should the naming convention be for the namespace (`mod` above)?
-
 .. note::
 
     This is inspired by [NEP 37](https://numpy.org/neps/nep-0037-array-module.html#how-to-use-get-array-module),
@@ -65,7 +63,7 @@ TBD: what should the naming convention be for the namespace (`mod` above)?
     standard can become final. We prefer to delay this decision, to see how
     NEP 37 adoption will work out.
 
-The `mod` namespace must contain the array object and all functionality
+The `xp` namespace must contain the array object and all functionality
 specified in :ref:`api-specification`. It may contain other functionality,
 however it is recommended not to add other functions or objects, because that
 may make it harder for users to write code that will work with multiple array