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3.0c3 Manual (as with 2.x):
os.extsep 
The character which separates the base filename from the extension; for
example, the '.' in os.py. Also available via os.path.
3.0c3
>>> import os
>>> os.extsep
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'extsep'
>>> os.path.extsep
'.'
os.extsep apparently was present in 2.x
Mention of os.extsep was also present in 2.x docstring and removed in
3.0 docstring, so perhaps this was intentional, but it does not make
much sense, so I am provisionally marking this as a library bug.

For more information: in 3.0, os.curdir, os.pardir, os.sep, os.altsep,
os.pathsep, os.defpath, and os.devnull are also in os.path. Only
os.extset was removed from os. Among the sysinfo constants, only
os.linesep was not in os.path. I think either all names or none should
be duplicated.
The doc for os.path begins "This module implements some useful functions
on pathnames". The duplication of the sysinfo constants is not
mentioned! (Though perhaps is should be "Also, as a convenience, the
system information constants from os are available here also.") So if
they are only in one place, it should be os, not os.path.
In any case, the removal of extpath and only that from os is gratuitous
breakage.

Re-added os.extsep to the os module in r67552.