pmb.parse.parse_attribute: strip off trailing comment on attrib line

The apkbuild parser could not handle cases where a line ends in a comment but the value is not quoted.

E.g. this:

pkgver=1.0 # I'm a comment, look at me

was being parsed to a value like this:

1.0 # I'm a comment, look at me

... which is obviously wrong. This strips off any trailing comment on the line, so it's parsed to the correct value.

fixes #2087 (closed)