Process for upgrading the stdlib to a new cpython version

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(Based on the method used by PyPy.)

0. Update a CPython checkout to latest.

1. git checkout python-stdlib

2. ./Src/StdLib/upgrade-stdlib.sh ~/path/to/cpython

3. git checkout -b update-stdlib master

4. git merge --no-ff python-stdlib

5. Fix conflicts, etc.

6. commit

7. git checkout master

8. git merge update-stdlib && git branch -d update-stdlib