[compat] Clarify the versions requiring compat_kwargs

It's supported since 2.7.0 alpha 1 and 2.6.5 rc 1. See
https://hg.python.org/cpython/file/v2.7a1/Misc/NEWS#l337
https://hg.python.org/cpython/file/v2.6.5rc1/Misc/NEWS#l28

youtube_dl/compat.py

@@ -433,7 +433,7 @@ if sys.version_info < (3, 0) and sys.platform == 'win32':
 else:
     compat_getpass = getpass.getpass
 
-# Old 2.6 and 2.7 releases require kwargs to be bytes
+# Python < 2.6.5 require kwargs to be bytes
 try:
     def _testfunc(x):
         pass