Bug 1780880 - Use util.stringio instead of pycompat.stringio. r=sheehan

pycompat.stringio was removed in Mercurial 6.2. util.stringio exists
for backward compatibility as it was the preferred symbol.

Differential Revision: https://phabricator.services.mozilla.com/D152572

--HG--
extra : moz-landing-system : lando

hgext/mozext/__init__.py

@@ -1219,7 +1219,7 @@ def apply_change(node, reverse, push_patch=True, name=None):
 
         opts = mdiff.defaultopts
         opts.git = True
-        rpatch = pycompat.stringio()
+        rpatch = util.stringio()
         orig, mod = (node, p1) if reverse else (p1, node)
         for chunk in patch.diff(repo, node1=orig, node2=mod, opts=opts):
             rpatch.write(chunk)