Avoid respecifying default encoding for .encode()/.decode() calls

In Python 3, both .encode() and .decode() default the encoding to
'utf-8'. See the docs:

https://docs.python.org/3/library/stdtypes.html#str.encode
https://docs.python.org/3/library/stdtypes.html#bytes.decode

Simplify and shorten the code by using the default instead of
respecifying it.

tests/test_setup_command.py

@@ -99,7 +99,7 @@ def nonascii_srcdir(request, setup_command):
                             .. toctree::
 
                                %(mb_name)s/%(mb_name)s
-                            """ % locals())).encode('utf-8'))
+                            """ % locals())).encode())
 
 
 @pytest.mark.usefixtures('nonascii_srcdir')