Add Python version since deprecation in base64 methods. (#33) (#429)

Allow developers to not have to either test on N Python versions or
looked through multiple versions of the docs to know whether they can
easily update.

(cherry picked from commit c643a967dd7d33ccefa5b61b38caf40b448057ce)
diff --git a/Lib/base64.py b/Lib/base64.py
index 58f6ad6..eb8f258 100755
--- a/Lib/base64.py
+++ b/Lib/base64.py
@@ -541,7 +541,8 @@
 def encodestring(s):
     """Legacy alias of encodebytes()."""
     import warnings
-    warnings.warn("encodestring() is a deprecated alias, use encodebytes()",
+    warnings.warn("encodestring() is a deprecated alias since 3.1, "
+                  "use encodebytes()",
                   DeprecationWarning, 2)
     return encodebytes(s)
 
@@ -554,7 +555,8 @@
 def decodestring(s):
     """Legacy alias of decodebytes()."""
     import warnings
-    warnings.warn("decodestring() is a deprecated alias, use decodebytes()",
+    warnings.warn("decodestring() is a deprecated alias since Python 3.1, "
+                  "use decodebytes()",
                   DeprecationWarning, 2)
     return decodebytes(s)