Switch from python2 to python.
Apply upstream recommendations for deprecation of dependencies
(especially the use of fonts).
Remove patch for CVE-2020-17353 as it is now included.
Disable the use of texihtml using configure.
Remove build time copies of fonts and rely on optdepends instead.

Change pkgdesc to upstream description.
Switch url to https.
Add all used licenses: FDL1.3, GPL3, OFL.
Add to pro-audio group.
Add all available soname deps in package() and the respective packages to makedepends.
Add all direct dependencies to depends.
Remove lilyfontsize.patch which was only added due to a misbehaving desktop environment.
Add upstream patch for CVE-2020-17353 (FS#67680).
Run autoconf in prepare() instead of autogen.sh (all heuristics in there are not needed).
Install custom license and docs.
Update maintainer info.

FTR, commands used to change arch=() and then verify the changes:

$ sed -Ei "/^arch=/{s/ *?['\"]?i686['\"]? *?//;s/ *$//}" */*/PKGBUILD*

$ grep -r i686 * | grep arch= | grep -v repos

$ svn diff | grep -A 1 ^-[^-] | grep -v -- -- | cut -c2- |
  while IFS=$'\n' read -r l1; IFS=$'\n' read -r l2; do
    echo "$l1 -> $l2"
  done | sort -u