Lift GitPython into 3.x version series

Some dependabot alerts directed the attention to the upper version limit
of GitPython. This excludes the currently used 3.1 releases which
neither makes sense nor helps installing fixed upstream dependencies via
pip. Therefore, switch this dependency to 3.1 or later.

Signed-off-by: Jan Kiszka <[email protected]>

Author: jan-kiszka

setup.py

@@ -95,7 +95,7 @@
         'distro>=1.0.0,<2',
         'jsonschema>=2.5.0,<5',
         'kconfiglib>=14.1.0,<15',
-        'GitPython>=2.1.11,<3'
+        'GitPython>=3.1.0,<4'
     ],
 
     # At least python 3.6 is needed by now: