I don't see a license information in the Tk distribution, nor in the documentation of the Tk module.

Your best bet is to contact the maintainer (he's know to be very responsive) and ask for a clarification of the license.

BTW you have to check the license of every module that you use, not just Tk.

Update: Found it now, it was just in a non-standard location. It's ini Tk/license.terms and says:

The authors hereby grant permission to use, copy, modify, distribute, and license this software and its documentation for any purpose, provided that existing copyright notices are retained in all copies and that this notice is included verbatim in any distributions. No written agreement, license, or royalty fee is required for any of the authorized uses. Modifications to this software may be copyrighted by their authors and need not follow the licensing terms described here, provided that the new terms are clearly indicated on the first page of each file where they apply.