ci: bump flake8 to new version

```
./electrum/commands.py:144:9: F824 `global known_commands` is unused: name is never assigned in scope
        global known_commands
        ^
./electrum/commands.py:1916:9: F824 `global known_commands` is unused: name is never assigned in scope
        global known_commands
        ^
./electrum/gui/qt/main_window.py:2405:13: F824 `nonlocal done` is unused: name is never assigned in scope
            nonlocal done
            ^
./electrum/i18n.py:52:5: F824 `global language` is unused: name is never assigned in scope
    global language
    ^
./electrum/plugin.py:189:9: F824 `global _root_permission_cache` is unused: name is never assigned in scope
        global _root_permission_cache
        ^
5     F824 `global known_commands` is unused: name is never assigned in scope
5

```

electrum/commands.py

@@ -141,8 +141,6 @@ class Command:
 
 def command(s):
     def decorator(func):
-        global known_commands
-
         if hasattr(func, '__wrapped__'):
             # plugin command function
             name = func.plugin_name + '_' + func.__name__
@@ -1913,7 +1911,6 @@ def plugin_command(s, plugin_name):
     """Decorator to register a cli command inside a plugin. To be used within a commands.py file
     in the plugins root."""
     def decorator(func):
-        global known_commands
         assert len(plugin_name) > 0, "Plugin name must not be empty"
         func.plugin_name = plugin_name
         name = plugin_name + '_' + func.__name__

electrum/gui/qt/main_window.py

@@ -2402,7 +2402,6 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger, QtEventListener):
             done = True
 
         def on_dialog_closed(*args):
-            nonlocal done
             nonlocal cancelled
             if not done:
                 cancelled = True

electrum/i18n.py

@@ -49,7 +49,6 @@ language = gettext.translation('electrum', fallback=True, class_=gettext.NullTra
 def _(msg: str, *, context=None) -> str:
     if msg == "":
         return ""  # empty string must not be translated. see #7158
-    global language
     if context:
         contexts = [context]
         if context[-1] != "|":  # try with both "|" suffix and without

electrum/plugin.py

@@ -186,7 +186,6 @@ class Plugins(DaemonThread):
     @profiler(min_threshold=0.5)
     def _has_recursive_root_permissions(self, path):
         """Check if a directory and all its subdirectories have root permissions"""
-        global _root_permission_cache
         if _root_permission_cache.get(path) is not None:
             return _root_permission_cache[path]
         _root_permission_cache[path] = False