Upgrade to new upstream release 0.16.0

To create these changes, I did:

# download orig.tar.gz
git-dpm import-new-upstream --rebase ../python-future_0.16.0.orig.tar.gz
# fix conflicts
git-dpm update-patches
pristine-tar commit ../python-future_0.16.0.orig.tar.gz upstream
git-dpm dch -- -v 0.16.0-1 "new upstream version 0.16.0"

So there are also corresponding upstream and pristine-tar branches in my fork of this repository.

Note that this resolves https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=863215.

I ran the tests and did not see any failures: https://salsa.debian.org/saraedum-guest/python-future/-/jobs/14758

I am new to debian/python-future, so I am very happy to get any feedback :)