Remove use of FILE_CHARSET setting in Django 3.1

The FILE_CHARSET setting is removed.

Author: browniebroke

django_coverage_plugin/plugin.py

@@ -134,7 +134,11 @@ def read_template_source(filename):
         settings.configure()
 
     with open(filename, "rb") as f:
-        text = f.read().decode(settings.FILE_CHARSET)
+        if django.VERSION >= (3, 1):
+            charset = 'utf-8'
+        else:
+            charset = settings.FILE_CHARSET
+        text = f.read().decode(charset)
 
     return text