Fix SyntaxWarning's

Fix SyntaxWarning's

Individual changes:
1) abc2ly: Fix SyntaxWarning

This line still works in Python 3.8, but it throws a warning:
> SyntaxWarning: "is" with a literal. Did you mean "=="?

2) musicxml2ly: Remove drummode

This code is fundamentally broken: NoteEvent::ly_expression() and
NoteEvent::chord_element_ly() miss a '%' to format the string. Instead
the code "calls" the string which Python meets with a SyntaxWarning.

Trying to hit this code path, musicxml only sets the drum_type attribute
in Note::initialize_drum_event() called from Note::to_lily_object().
However as far as I understand the MusicXML specification, a <note>
must always have either a <pitch>, or <unpitched>, or <rest>.
If I try to construct an (invalid) input with only an instrument name,
musicxml2ly breaks at another function, so this is really dead code.

[dak, 2020-02-02 15:44:48]

> However as far as I understand the MusicXML specification, a <note>
must always have either a <pitch>, or <unpitched>, or <rest>.

Wouldn't percussion notes generally be "unpitched"?  What happens for such
notes?

[hahnjo, 2020-02-02 15:47:44]

On 2020/02/02 15:44:48, dak wrote:
> > However as far as I understand the MusicXML specification, a <note>
> must always have either a <pitch>, or <unpitched>, or <rest>.
> 
> Wouldn't percussion notes generally be "unpitched"?  What happens for such
> notes?

Yes, they are in valid MusicXML. This is handled in
initialize_unpitched_event(), see context for python/musicxml.py

[hahnjo, 2020-02-02 15:49:36]

Drop drumtype_dict