text: Title: Method annotations incorrectly decorate inherited consumer methods
  Body: ## Precondition
  
  Consider the following consumer class:
  ```python
  class GitHub(uplink.Consumer):
  
      @uplink.get("/users/{username}")
      def get_user(self, username):
          """Get a single user."""
  ```
  
  Use any method annotation. For this example, I'll create a custom method annotation, `PrintMethodName`, that simply prints the name of the consumer method that it wraps:
  
  ```python
  from uplink import decorators
  
  class PrintMethodName(decorators.MethodAnnotation):
  
      def modify_request_definition(self, method):
          print(method.__name__)
  ```
  
  ## Steps to recreate
  
  Create a subclass of `GitHub` and decorate it with the method annotation:
  ```python
  @PrintMethodName()
  class GitHubSubclass(GitHub):
      pass
  ```
  
  ## Expected
  
  No output to stdout, since `GitHubSubclass` doesn't define any consumer methods.
  
  ## Actual
  
  The method annotation decorates `get_user` from the parent class, `GitHub`.
  
  Output:
  ```python
  get_user
  ```