Reduce monkeypatching in pyflakes checker (#372)

Chipping away at #183 

Got rid of the monkeypatched `LAMBDA`.

What's changed ->
- Default argument values cannot be forward references anymore:

```python
def bar(x: type = SomeClass) -> None: ...  # F821 undefined name 'SomeClass'
class SomeClass: ...
```
I think it makes sense to not allow this anyway - it's similar to what
we did with classes previously.

CHANGELOG.md

@@ -1,5 +1,17 @@
 # Change Log
 
+## Unreleased
+
+* The way in which flake8-pyi modifies pyflakes runs has been improved. When
+  flake8-pyi is installed, pyflakes will now complain about forward references
+  in default values for function and method parameters (the same as pyflakes
+  does when it checks `.py` files). Unlike in `.py` files, forward references
+  in default values are legal in stub files. However, they are never necessary,
+  and are considered bad style. (Forward references for parameter *annotations*
+  are still allowed.)
+
+  Contributed by [tomasr8](https://github.com/tomasr8).
+
 ## 23.5.0
 
 * flake8-pyi no longer supports being run with flake8 <5.0.4.

pyi.py

@@ -216,17 +216,6 @@ def ASSIGN(
 
         self.deferHandleNode(tree.value, tree)
 
-    def LAMBDA(self, node: ast.Lambda) -> None:
-        """This is likely very brittle, currently works for pyflakes 1.3.0.
-
-        Deferring annotation handling depends on the fact that during calls
-        to LAMBDA visiting the function's body is already deferred and the
-        only eager calls to `handleNode` are for annotations.
-        """
-        self.handleNode, self.deferHandleNode = self.deferHandleNode, self.handleNode  # type: ignore[method-assign]
-        super().LAMBDA(node)
-        self.handleNode, self.deferHandleNode = self.deferHandleNode, self.handleNode  # type: ignore[method-assign]
-
     def handleNodeDelete(self, node: ast.AST) -> None:
         """Null implementation.
 

tests/forward_refs.pyi

@@ -21,3 +21,11 @@ class C:
 class Outer:
     class Inner1: ...
     class Inner2(list[Outer.Inner1]): ...
+
+# Allow forward references for argument annotations and the return type
+def foo(x: SomeClass) -> SomeClass: ...
+class SomeClass: ...
+
+# Disallow forward references for default argument values
+def bar(x: type = SomeOtherClass) -> None: ...  # Y011 Only simple default values allowed for typed arguments  # F821 undefined name 'SomeOtherClass'
+class SomeOtherClass: ...