py3: use b"%d" instead of str() to convert int to bytes

py3: use b"%d" instead of str() to convert int to bytes

While I was here, I added 'and None' to suppress return values of .write()
calls.

Differential Revision: https://phab.mercurial-scm.org/D3328

--- a/tests/test-clone-uncompressed.t	Sat Apr 14 02:03:02 2018 +0530
+++ b/tests/test-clone-uncompressed.t	Sat Apr 14 02:04:19 2018 +0530
@@ -18,7 +18,7 @@
   $ hg -q commit -A -m initial
   >>> for i in range(1024):
   ...     with open(str(i), 'wb') as fh:
-  ...         fh.write(str(i))
+  ...         fh.write(b"%d" % i) and None
   $ hg -q commit -A -m 'add a lot of files'
   $ hg st
   $ hg --config server.uncompressed=false serve -p $HGPORT -d --pid-file=hg.pid