Encoding preparations for Python 3.5

Encoding preparations for Python 3.5

1. Use codecs.open() to decode as utf-8

This is in preparation for Python 3.5 where the default encoding
depends on the value of the LANG environment variable. As far as
I can tell, this was changed later on and at least Python 3.7 and
version 3.8 always default to 'utf-8' on Linux. As I'm proposing to
make Python 3.5 the required minimum, we can't rely on this and need
to force 'utf-8' when reading files that could contain Unicode.

2. musicxml2ly: Correct usage of codecs module

sys.stdout is already in lilylib and codecs.open() always opens
a file in binary mode.

3. lilylib: Remove encoded_write

It's not needed anymore as sys.stdout and sys.stderr are wrapped
with encoding writers. (The argument file for show_rules in script
convert-ly is also sys.stdout.)

[hahnjo, 2020-01-17 08:52:35]

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