Bundle misc/proper_plugin.py as a part of mypy (#16036)

I modified 

```diff
diff --git mypy/binder.py mypy/binder.py
index 8a68f24f6..194883f86 100644
--- mypy/binder.py
+++ mypy/binder.py
@@ -345,7 +345,7 @@ class ConditionalTypeBinder:
             self._cleanse_key(dep)
 
     def most_recent_enclosing_type(self, expr: BindableExpression, type: Type) -> Type | None:
-        type = get_proper_type(type)
+        # type = get_proper_type(type)
         if isinstance(type, AnyType):
             return get_declaration(expr)
         key = literal_hash(expr)
```

to see if it still works. It is:

```python
» python runtests.py self
run self: ['/Users/sobolev/Desktop/mypy/.venv/bin/python', '-m', 'mypy', '--config-file', 'mypy_self_check.ini', '-p', 'mypy', '-p', 'mypyc']
mypy/binder.py:349: error: Never apply isinstance() to unexpanded types; use
mypy.types.get_proper_type() first  [misc]
            if isinstance(type, AnyType):
               ^~~~~~~~~~~~~~~~~~~~~~~~~
mypy/binder.py:349: note: If you pass on the original type after the check, always use its unexpanded version
Found 1 error in 1 file (checked 288 source files)

FAILED: self
```

I will add this plugin to my CI checks in like ~5 plugins I maintain :)

- https://github.com/typeddjango/django-stubs
- https://github.com/typeddjango/djangorestframework-stubs
- https://github.com/dry-python/returns
- https://github.com/dry-python/classes
- https://github.com/wemake-services/mypy-extras

Closes #16035

MANIFEST.in

@@ -34,7 +34,6 @@ include build-requirements.txt
 include test-requirements.txt
 include mypy_self_check.ini
 prune misc
-include misc/proper_plugin.py
 graft test-data
 include conftest.py
 include runtests.py

docs/source/extending_mypy.rst

@@ -237,3 +237,12 @@ mypy's cache for that module so that it can be rechecked. This hook
 should be used to report to mypy any relevant configuration data,
 so that mypy knows to recheck the module if the configuration changes.
 The hooks should return data encodable as JSON.
+
+Useful tools
+************
+
+Mypy ships ``mypy.plugins.proper_plugin`` plugin which can be useful
+for plugin authors, since it finds missing ``get_proper_type()`` calls,
+which is a pretty common mistake.
+
+It is recommended to enable it is a part of your plugin's CI.

misc/proper_plugin.py → mypy/plugins/proper_plugin.py

@@ -1,3 +1,12 @@
+"""
+This plugin is helpful for mypy development itself.
+By default, it is not enabled for mypy users.
+
+It also can be used by plugin developers as a part of their CI checks.
+
+It finds missing ``get_proper_type()`` call, which can lead to multiple errors.
+"""
+
 from __future__ import annotations
 
 from typing import Callable

mypy_self_check.ini

@@ -5,7 +5,7 @@ disallow_any_unimported = True
 show_traceback = True
 pretty = True
 always_false = MYPYC
-plugins = misc/proper_plugin.py
+plugins = mypy.plugins.proper_plugin
 python_version = 3.8
 exclude = mypy/typeshed/|mypyc/test-data/|mypyc/lib-rt/
 new_type_inference = True