Removed support for tkinter in PyPy before Python 3.6

[nulano]

Continuing from #6549, this PR removes support for old PyPy.

The function interpaddr was added to PyPy over 4 years ago: https://foss.heptapod.net/pypy/pypy/-/commit/8f15f623274f84bae5a4c3be914abb96b604a254

From the tags on that commit, I can see it was included in release-pypy3.6-v7.0.0, the first Python 3.6 PyPy release.
Since Python 3.5 is no longer supported by Pillow, the old code can be removed without deprecation.

[hugovk]

👍

There's also some PyPy3.5 tags on that commit.

And to look at it from the other angle: Python 3.7 is the oldest version we support.

PyPy3.7 stable was released on 2021-04-08, a good few years after the 2017-11-05 commit.

(We don't support pre-release PyPy, but PyPy3.7 alpha was 2020-09-25 and beta was 2020-11-21, also both long after the commit.)