patman: Use unicode for file I/O

At present patman test fail in some environments which don't use utf-8
as the default file encoding. Add this explicitly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/series.py b/tools/patman/series.py
index d667d9b..02a1113 100644
--- a/tools/patman/series.py
+++ b/tools/patman/series.py
@@ -223,7 +223,7 @@
         col = terminal.Color()
         # Look for commit tags (of the form 'xxx:' at the start of the subject)
         fname = '/tmp/patman.%d' % os.getpid()
-        fd = open(fname, 'w')
+        fd = open(fname, 'w', encoding='utf-8')
         all_ccs = []
         for commit in self.commits:
             cc = []