util: use pycompat.bytestr() instead of str()

This fixes at least some environment variable prints for util.system()
callers on Python 3. Yay!

Differential Revision: https://phab.mercurial-scm.org/D2434

mercurial/util.py

@@ -1188,7 +1188,7 b' def shellenviron(environ=None):'
             return '0'
         if val is True:
             return '1'
-        return str(val)
+        return pycompat.bytestr(val)
     env = dict(encoding.environ)
     if environ:
         env.update((k, py2shell(v)) for k, v in environ.iteritems())