core: removed sat_tmp.twisted patches + update minimum Twisted version:

- `sat_tmp` patches are not needed anymore and cause trouble with latest Twisted (21.02)
- update minimum to `20.3.0` which is the first one to support Python 3.8

setup.py

@@ -44,7 +44,7 @@ install_requires = [
     'pyxdg',
     'sat_tmp >= 0.7.0a4',
     'shortuuid',
-    'twisted[tls] >= 19.7.0',
+    'twisted[tls] >= 20.3.0',
     'treq',
     'urwid >= 1.2.0',
     'urwid-satext >= 0.7.0a2',

twisted/plugins/sat_plugin.py

@@ -26,12 +26,6 @@ from twisted.application.service import IServiceMaker
 from sat.core.constants import Const as C
 from sat.core.i18n import _
 
-# patch for Python 3.8 compatibility
-from sat_tmp.twisted import install as install_twisted_patches
-
-
-install_twisted_patches()
-
 
 def initialise(options):
     """Method to initialise global modules"""