Fix some more str/bytes issues.

Turns out, the source of a DTML document should be text, and only
when rendered out over the publisher be turned into bytes.

src/DocumentTemplate/DT_String.py

@@ -292,7 +292,7 @@ def parse_close(self, text, start, tagre, stag, sloc, scommand, sa):
     shared_globals__roles__ = ()
     shared_globals = {}
 
-    def __init__(self, source_string=b'', mapping=None, __name__='<string>',
+    def __init__(self, source_string='', mapping=None, __name__='<string>',
                  **vars):
         """\
         Create a document template from a string.

src/TreeDisplay/TreeTag.py

@@ -614,7 +614,7 @@ def encode_seq(state):
         state = state[:l]
 
     state = state.translate(tplus)
-    return state
+    return state.decode('ascii')
 
 
 def encode_str(state):
@@ -729,4 +729,4 @@ def tpValuesIds(self, get_items, args,
 
 
 def oid(self):
-    return b2a_base64(str(self._p_oid))[:-1]
+    return b2a_base64(self._p_oid)[:-1].decode('ascii')