fix: generate py39 wheels too

The absence of 
soundswallower-0.6.4-cp39-cp39-win_amd64.whl on pypi means I (and therefore Studio CLI users) cannot install ReadAlongs/Studio on my windows machine using Python 3.9 without installing the MS Visual Studio compiler.
So add py39 wheels to pyproject.toml so they're generated next time we release.

pyproject.toml

@@ -42,6 +42,7 @@ soundswallower = "soundswallower.cli:main"
 # Build a reduced selection of binaries as there are tons of them
 build = [
       "cp38-*",
+      "cp39-*",
       "cp310-*",
       "cp311-*",
       "cp312-*",