scanpypi: write every license file once

scanpypi: write every license file once

On some cases, when the package contains multiple license files
and some of them from the same type, the scanpypi script will write
the same license type more than once under _LICENSE.
Hence, before creating the _LICENSE variable, we'll remove every
possible duplication.

Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>