Move mypy configuration to setup.cfg

Author: snejus

.mypy.ini

@@ -179,3 +179,9 @@ per-file-ignores =
     ./beets/dbcore/queryparse.py:D
     ./beets/dbcore/types.py:D
     ./beets/mediafile.py:D
+
+[mypy]
+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
+ignore_missing_imports = true