py3: convert popen() command arguments in hgclient to str on Windows

This fixes test-commandserver.t and test-keyword.t, which was previously
complaining

        TypeError("a bytes-like object is required, not 'str'")

contrib/hgclient.py

@@ -33,7 +33,12 b' def connectpipe(path=None, extraargs=())'
         cmdline += [b'-R', path]
     cmdline.extend(extraargs)
 
-    server = subprocess.Popen(cmdline, stdin=subprocess.PIPE,
+    def tonative(cmdline):
+        if os.name != r'nt':
+            return cmdline
+        return [arg.decode("utf-8") for arg in cmdline]
+
+    server = subprocess.Popen(tonative(cmdline), stdin=subprocess.PIPE,
                               stdout=subprocess.PIPE)
 
     return server