hg-docker: fix Python 3.4 compatibility (for CentOS 7)

I realize Mercurial is not targetting Python 3.4 compatibility,
but without this change, it's not even possible to build it on
CentOS 7 (and I assume the same is true for RHEL 7).

Differential Revision: https://phab.mercurial-scm.org/D5570

contrib/packaging/hg-docker

@@ -72,7 +72,12 b' def build_docker_image(dockerfile: pathl'
     ]
 
     print('executing: %r' % args)
-    subprocess.run(args, input=dockerfile, check=True)
+    p = subprocess.Popen(args, stdin=subprocess.PIPE)
+    p.communicate(input=dockerfile)
+    if p.returncode:
+        raise subprocess.CalledProcessException(
+                p.returncode, 'failed to build docker image: %s %s' \
+                % (p.stdout, p.stderr))
 
 def command_build(args):
     build_args = []