Fixes #4493.

I found a "clang-format style that approximates Python's PEP 7":

• https://github.com/pganssle/zoneinfo/blob/master/.clang-format

(The zoneinfo repo was the reference implementation for zoneinfo in the stdlib (PEP 615), and is now a backport for Python 3.6+.)

This style has also been used elsewhere, as is or as the basis:

• https://github.com/search?q=%22A+clang-format+style+that+approximates+Python%27s+PEP+7%22&type=Code

Including CPython's new PEG generator (written by Guido and two others), which is the same as the zoneinfo one but with ColumnLimit increased from 79 to 95:

• https://github.com/python/cpython/blob/master/Tools/peg_generator/.clang-format

Here's how it looks applied to Pillow:

clang-format -i `git ls-tree -r HEAD --name-only | grep "\.[ch]$"`

1. The first commit is the same config file as in zoneinfo
2. The second formats the codebase with the config
3. The third increases ColumnLimit from 79 to 88, to match Black
4. The fourth formats again with the 88 limit

I suggest we use this, and welcome feedback and any further tweaks.

Before merge, to make the history cleaner, I'll squash the config to a single commit, and the formatting to another single commit.

Also this will need adding to the CI.