Explictly attribute other licenses

Author: tekktrik

README.rst

@@ -140,3 +140,8 @@ Documentation
 
 For information on building library documentation, please check out
 `this guide <https://learn.adafruit.com/creating-and-sharing-a-circuitpython-library/sharing-our-docs-on-readthedocs#sphinx-5-1>`_.
+
+Attribution
+===========
+
+Some code contained here is ported from CPython, dual licensed by the Python Software Foundation under the PSF License verion 2 and the Zero-Clause BSD license.

circuitpython_csv.py

@@ -2,6 +2,9 @@
 # SPDX-FileCopyrightText: Copyright (c) 2021 Alec Delaney
 #
 # SPDX-License-Identifier: MIT
+# SPDX-License-Identifier: PSF-2.0
+# SPDX-License-Identifier: 0BSD
+
 """
 `circuitpython_csv`
 ================================================================================