As 3.9 has never officially been released and instead was skipped over straight to 4.0, the 3.9 changelog section can be seen as "3.9 and 4.0". I have increased it to 4.0 to prevent any further version confusion.
Strangely I misremembered that we already increased this heading's version number to 4.0 before, but I must have confused that with something else. Thanks for the hint!