Always respect the shebangs of LilyPond's Python tools on macOS

[jeanas]

According to the docstrings, a system Python was searched because the LilyPond-provided one did not function correctly. With current versions of LilyPond, which use a completely rewritten system for generating the binaries instead of GUB, it does.

Fixes #1492

[dliessi]

All these changes are fine, but there is one missing: we should delete the preference useshebang by means of install/update.py.
Anyway @jeanas you can already merge this PR: I'm going to add a follow-up commit (here if before you merge it, otherwise directly to master).

I am in favour of this change, but for the record I want to point out that this change may break the use of LilyPond's Python tools for older versions of LilyPond.
In my opinion this is not a problem, as convert-ly and imports usually make sense only with a recent version of LilyPond, if not the latest, and this PR has no effect on running LilyPond proper.

[jeanas]

Good remark about install/update.py, I was starting to wonder about it. Given that it won't harm although it won't be used, can we just leave it there or should we delete it?

[dliessi]

You are right that unused preferences don't hurt, unless the same name is reused at a later time (the old preference value would be picked up). Even though that's only a theoretical risk, I would prefer to delete the unused preference entry.

[jeanas]

OK, understood. I have added a commit that does this.

[fedelibre]

Jean, I think you can merge it.

I don't see any other open PR ready to be included in tomorrow release.
Let's release 3.3.0 tomorrow, then we can make a bug fix release (3.3.1) in a few weeks to include other stuff.

I'll make the release tomorrow in the afternoon, Ok?
Then I would wait until Mac and Windows packages are ready (if it doesn't take too long) before announcing the release in the mailing lists.

[jeanas]

Right now, I'm installing things in order to test this on @marnen's MacStadium node. If it looks OK, I will merge it.

The only remaining bug that I'd hope to fix before tomorrow is #1419 -- it's quite unfriendly for a newcomer to have an exception thrown if they try to compile a file before they understand that they must install LilyPond separately -- but it can also go into a bugfix release, no need to delay 3.3 longer.

[jeanas]

#1567 is a simple fix for #1419.

[jeanas]

The test worked fine. Let's merge this now.