Fix packing on new charmcraft versions

New minor release of charmcraft requires us to change the code, as it
introduced a breaking change. This patch adds git and updates setuptools
before packing the charm.

See canonical/charmcraft#551 for discussion

charmcraft.yaml

@@ -8,3 +8,6 @@ bases:
       channel: "20.04"
     - name: "centos"
       channel: "7"
+parts:
+  charm:
+    charm-python-packages: [setuptools]