Fix typos and wording in what’s new 3.2.

Fix typos and wording in what’s new 3.2.

- The entry about shutil.copytree is just a revert of r84524 which
looks like an unfinished edition.
- The use of gender-neutral language (s/his/their/) removes the
implicit assumption that programmer == male (change agreed by Antoine).
- Other changes should be uncontroversial fixes.

I haven’t rewrapped under 80 lines to keep the diffs readable; I’ll
rewrap later.