py3: suppress DeprecationWarning about deprecated base64 module aliases stable

py3: suppress DeprecationWarning about deprecated base64 module aliases

base64.encodestring() / base64.decodestring() were renamed to
base64.encodebytes() / base64.decodebytes() in Python 3. The old names still
worked, but raised a DeprecationWarning.

--- a/hgext/convert/common.py	Mon Jun 15 03:38:02 2020 +0200
+++ b/hgext/convert/common.py	Tue Jun 16 12:59:45 2020 +0200
@@ -84,9 +84,17 @@
     return l
 
 
+if pycompat.ispy3:
+    base64_encodebytes = base64.encodebytes
+    base64_decodebytes = base64.decodebytes
+else:
+    base64_encodebytes = base64.encodestring
+    base64_decodebytes = base64.decodestring
+
+
 def encodeargs(args):
     def encodearg(s):
-        lines = base64.encodestring(s)
+        lines = base64_encodebytes(s)
         lines = [l.splitlines()[0] for l in pycompat.iterbytestr(lines)]
         return b''.join(lines)
 
@@ -95,7 +103,7 @@
 
 
 def decodeargs(s):
-    s = base64.decodestring(s)
+    s = base64_decodebytes(s)
     return pickle.loads(s)