fix: correct string interpolation for PyPI download URL

The download URL under the "Project links" section on the PyPI site [1]
is currently broken, as it points to the following URL:

https://github.com/siemens/kas/archive/{__version__}.tar.gz

This commit fixes the issue by adding the 'f' prefix to correctly
interpolate the __version__ variable in the URL.

[1] https://pypi.org/project/kas/

Signed-off-by: Michael Adler <michael.adler@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

setup.py

@@ -50,7 +50,7 @@
 
     url='https://github.com/siemens/kas',
     download_url=('https://github.com/siemens/'
-                  'kas/archive/{__version__}.tar.gz'),
+                  f'kas/archive/{__version__}.tar.gz'),
 
     license='MIT',