golang_str: Move py3/py2 conditioning into _utf8_{encode,decode}_surrogateescape

So that those routines could be just called and do what is expected
without the caller caring whether it is py2 or py3. We will soon need to
use those routines from several callsites, and having that py2/py3
conditioning being spread over all usage places would be inconvenient.

/reviewed-by @jerome
/reviewed-at !18

golang/_golang_str.pyx

@@ -43,13 +43,7 @@ def pyb(s): # -> bytes
     if isinstance(s, bytes):                    # py2: str      py3: bytes
         pass
     elif isinstance(s, unicode):                # py2: unicode  py3: str
-        if PY_MAJOR_VERSION >= 3:
-            s = s.encode('UTF-8', 'surrogateescape')
-        else:
-            # py2 does not have surrogateescape error handler, and even if we
-            # provide one, builtin unicode.encode() does not treat
-            # \udc80-\udcff as error. -> Do the encoding ourselves.
-            s = _utf8_encode_surrogateescape(s)
+        s = _utf8_encode_surrogateescape(s)
     else:
         raise TypeError("b: invalid type %s" % type(s))
 
@@ -76,13 +70,7 @@ def pyu(s): # -> unicode
     if isinstance(s, unicode):                  # py2: unicode  py3: str
         pass
     elif isinstance(s, bytes):                  # py2: str      py3: bytes
-        if PY_MAJOR_VERSION >= 3:
-            s = s.decode('UTF-8', 'surrogateescape')
-        else:
-            # py2 does not have surrogateescape error handler, and even if we
-            # provide one, builtin bytes.decode() does not treat surrogate
-            # sequences as error. -> Do the decoding ourselves.
-            s = _utf8_decode_surrogateescape(s)
+        s = _utf8_decode_surrogateescape(s)
     else:
         raise TypeError("u: invalid type %s" % type(s))
 
@@ -243,6 +231,12 @@ def _utf8_decode_rune(s):
 # _utf8_decode_surrogateescape mimics s.decode('utf-8', 'surrogateescape') from py3.
 def _utf8_decode_surrogateescape(s): # -> unicode
     assert isinstance(s, bytes)
+    if PY_MAJOR_VERSION >= 3:
+        return s.decode('UTF-8', 'surrogateescape')
+
+    # py2 does not have surrogateescape error handler, and even if we
+    # provide one, builtin bytes.decode() does not treat surrogate
+    # sequences as error. -> Do the decoding ourselves.
     outv = []
     emit = outv.append
 
@@ -276,6 +270,12 @@ def _utf8_decode_surrogateescape(s): # -> unicode
 # _utf8_encode_surrogateescape mimics s.encode('utf-8', 'surrogateescape') from py3.
 def _utf8_encode_surrogateescape(s): # -> bytes
     assert isinstance(s, unicode)
+    if PY_MAJOR_VERSION >= 3:
+        return s.encode('UTF-8', 'surrogateescape')
+
+    # py2 does not have surrogateescape error handler, and even if we
+    # provide one, builtin unicode.encode() does not treat
+    # \udc80-\udcff as error. -> Do the encoding ourselves.
     outv = []
     emit = outv.append