Add python 3.9

add a changelog entry for #1124

- add a changelog entry for #1124
- correct duplicate entry for 3.1.12 -> 3.1.13

add replace method to git.Commit

This adds a replace method to git.Commit. The replace method returns a
copy of the Commit object with attributes replaced from keyword
arguments. For example:

>>> old = repo.head.commit
>>> new = old.replace(message='This is a test')

closes #1123