Define paths to lint in mypy configuration

setup.cfg

@@ -181,6 +181,7 @@ per-file-ignores =
     ./beets/mediafile.py:D
 
 [mypy]
+files = beets,beetsplug,test
 allow_any_generics = false
 # FIXME: Would be better to actually type the libraries (if under our control),
 # or write our own stubs. For now, silence errors

tox.ini

@@ -27,8 +27,7 @@ passenv = INTEGRATION_TEST
 commands =
     test: python -m pytest {posargs}
     lint: python -m flake8 {posargs} {[_lint]files}
-    mypy: mypy -p beets -p beetsplug
-    mypy: mypy test
+    mypy: mypy
 
 [testenv:docs]
 basepython = python3.10