Add minimal plug-in

This plug-in is invoked twice, when analyzing `target.py`. Since it
always returns None, all it's results are ignored.

mypy.ini

@@ -15,3 +15,5 @@ warn_return_any = True
 no_implicit_reexport = True
 strict_equality = True
 strict_concatenate = True
+
+plugins = src/plugin.py

src/plugin.py

@@ -0,0 +1,13 @@
+from mypy.plugin import Plugin
+
+
+class CustomPlugin(Plugin):
+    def get_type_analyze_hook(self, fullname: str) -> None:
+        # see explanation below
+        if fullname.startswith("sol.GetItem"):
+            print(fullname)
+
+
+def plugin(version: str) -> type[CustomPlugin]:
+    # ignore version argument if the plugin works with all mypy versions.
+    return CustomPlugin

src/target.py

@@ -1,6 +1,8 @@
 from sol import GetItem
 
-reveal_type(GetItem())
+# reveal_type(GetItem())
 
-def f():
+_: GetItem
+
+def f(_: GetItem) -> None:
     ...