Prefer iter(dict) instead of dict.keys()

They are equivalent for iterating so remove the additional function
call.

Pattern identified as outdated by Lennart Regebro's PyCon 2017 talk
"Prehistoric Patterns in Python"

https://www.youtube.com/watch?v=V5-JH23Vk0I

src/flake8/main/application.py

@@ -261,7 +261,7 @@ class Application(object):
             List of filenames to process
         """
         if self.running_against_diff:
-            files = sorted(self.parsed_diff.keys())
+            files = sorted(self.parsed_diff)
         self.file_checker_manager.start(files)
         self.file_checker_manager.run()
         LOG.info('Finished running')

src/flake8/main/vcs.py

@@ -36,4 +36,4 @@ def install(option, option_string, value, parser):
 
 def choices():
     """Return the list of VCS choices."""
-    return list(_INSTALLERS.keys())
+    return list(_INSTALLERS)

src/flake8/plugins/_trie.py

@@ -90,7 +90,7 @@ class TrieNode(object):
         if not self.children:
             return
 
-        for prefix in sorted(self.children.keys()):
+        for prefix in sorted(self.children):
             child = self.children[prefix]
             yield child
             for child in child.traverse():

src/flake8/statistics.py

@@ -17,7 +17,7 @@ class Statistics(object):
         :rtype:
             list(str)
         """
-        return sorted({key.code for key in self._store.keys()})
+        return sorted({key.code for key in self._store})
 
     def record(self, error):
         """Add the fact that the error was seen in the file.
@@ -55,7 +55,7 @@ class Statistics(object):
         :returns:
             Generator of instances of :class:`Statistic`
         """
-        matching_errors = sorted(key for key in self._store.keys()
+        matching_errors = sorted(key for key in self._store
                                  if key.matches(prefix, filename))
         for error_code in matching_errors:
             yield self._store[error_code]

tests/unit/test_plugin_type_manager.py

@@ -205,7 +205,7 @@ class FakePluginTypeManager(manager.NotifierBuilderMixin):
 
     def __init__(self, manager):
         """Initialize with our fake manager."""
-        self.names = sorted(manager.keys())
+        self.names = sorted(manager)
         self.manager = manager