Deepfreeze essential parts of the encodings module. ### Description

This is a split off from #3582.

Deepfreeze essential parts of encodings module (encodings.\__init__, encodings.utf_8, and encodings.aliases), which is required during the Python bootstrap.

--- a/skimage/io/manage_plugins.py  2019-03-19 07:17:05.000000000 +0100
+++ b/skimage/io/manage_plugins.py	2020-12-16 12:03:52.000000000 +0100
@@ -38,7 +38,7 @@ plugin_meta_data = {}
 # the following preferences.
 preferred_plugins = {
     # Default plugins for all types (overridden by specific types below).
-    'all': ['imageio', 'pil', 'matplotlib', 'qt'],
+    'all': ['imageio', 'pil', 'matplotlib'],
     'imshow': ['matplotlib'],
     'imshow_collection': ['matplotlib']
 }
 
--- a/skimage/io/manage_plugins.py  2019-03-19 07:17:05.000000000 +0100
+++ b/skimage/io/manage_plugins.py	2020-12-16 12:03:52.000000000 +0100
@@ -38,7 +38,7 @@ plugin_meta_data = {}
 # the following preferences.
 preferred_plugins = {
     # Default plugins for all types (overridden by specific types below).
-    'all': ['imageio', 'pil', 'matplotlib', 'qt'],
+    'all': ['imageio', 'pil', 'matplotlib'],
     'imshow': ['matplotlib'],
     'imshow_collection': ['matplotlib']
 }
 