Exclude the github issue template from our sdist (#2440)

PR #2413 added an issue template, but just adding files to the project
directory upsets the packaging scripts: we need to explicitly include or
exclude them.

Move the template into a .github directory to make that easy, and to de-clutter
the root a bit.

MANIFEST.in

@@ -27,4 +27,5 @@ exclude jenkins*.sh
 exclude jenkins*
 recursive-exclude jenkins *.sh
 
+prune .github
 prune demo/etc