package/putty: security bump version to 0.73

Added upstream-provided sha1 hash.

Changelog:
https://www.chiark.greenend.org.uk/~sgtatham/putty/changes.html



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>