pyproject.toml: fix tests getting installed

Instead of automatic python package discovery that for some reason still
picks up the test module even though it should be excluded, specify the
"pmb" module manually to be installed.

Follow-up to MR 2551.

Fixes: issue 2541

pyproject.toml

@@ -25,8 +25,8 @@ Homepage = "https://www.postmarketos.org"
 [tool.setuptools.dynamic]
 version = {attr =  "pmb.__version__"}
 
-[tool.setuptools.packages.find]
-exclude = ["aports", "docs", "keys", "test", "test.pmb_test"]
+[tool.setuptools.package-dir]
+pmb = "pmb"
 
 [tool.ruff]
 line-length=100