gnu: snakemake-6: Convert dependencies to propagated-inputs

* gnu/packages/python-xyz.scm (snakemake-6)[inputs]: Change to ...
[propagated-inputs]: ... this, to ensure that "python -m snakemake"
works (because snakemake uses it internally).

Signed-off-by: Marius Bakke <marius@gnu.org>

gnu/packages/python-xyz.scm

@@ -9688,7 +9688,7 @@ Python style, together with a fast and comfortable execution environment.")
                ;; because there are no AWS credentials.
                (delete-file "tests/test_tibanna.py")
                (invoke "pytest")))))))
-    (inputs
+    (propagated-inputs
      (list python-appdirs
            python-configargparse
            python-connection-pool