Bug#906144: mypaint: Unable to install MyPaint when Gimp 2.10 is installed

[Jacob Nevins <jacobn+debian@chiark.greenend.org.uk>]

There's a patch (to mypaint rather than libmypaint) to resolve the
conflict without a new version of mypaint, by renaming mypaint's locale
to 'mypaint12', in the upstream tracker at
<https://github.com/mypaint/mypaint/issues/918#issuecomment-433645267>.

This is the approach previously suggested by Chris Waters in
<https://bugs.debian.org/894757#25>.

I haven't tested it myself, but it looks plausible.

(Cc'd the existing mypaint bug for this conflict,
<https://bugs.debian.org/897909>.)