py3: pass str into grp.getgrnam

grp.getgrnam expects str on Python 3.

This fixes test-acl.t on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D5794

contrib/python3-whitelist

@@ -5,6 +5,7 b' test-absorb-phase.t'
 test-absorb-rename.t
 test-absorb-strip.t
 test-absorb.t
+test-acl.t
 test-add.t
 test-addremove-similar.t
 test-addremove.t

mercurial/posix.py

@@ -583,6 +583,7 b' def groupmembers(name):'
     """Return the list of members of the group with the given
     name, KeyError if the group does not exist.
     """
+    name = pycompat.sysstr(name)
     return list(grp.getgrnam(name).gr_mem)
 
 def spawndetached(args):