Keep the LICENSE.txt file in lib/pythonX.Y dir
The license() builtin tries to read it and virtualenv tries to copy it.
Up until now, the license() builtin juts felt back to:
However it should output the full license text.
Virtualenv ~16.6 warns:
No LICENSE.txt / LICENSE found in source
Technically, it is probably possible to install the package without
%license files, but that would simply resort to the previous noncritical
This fix is not critical and hence it doesn't bump release, for easier
backporting to all our Python packages.