Modernize code to use f-strings

This also removes the encoding on the top, since Python 3 does it by
default. It also changes some methods to use `yield from`.

discord/file.py

@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-
 """
 The MIT License (MIT)
 
@@ -65,7 +63,7 @@ class File:
 
         if isinstance(fp, io.IOBase):
             if not (fp.seekable() and fp.readable()):
-                raise ValueError('File buffer {!r} must be seekable and readable'.format(fp))
+                raise ValueError(f'File buffer {fp!r} must be seekable and readable')
             self.fp = fp
             self._original_pos = fp.tell()
             self._owner = False