cleanup: Update codebase to expect Python 3.6

- Bump minimum version to Python 3.6.
- Use f-strings in a lot of places.

Change-Id: I2aa70197230fcec2eff8e7c8eb754f20c08075bb
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389034
Tested-by: Jason R. Coombs <jaraco@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

tests/test_manifest_xml.py

@@ -198,13 +198,13 @@ class ValueTests(unittest.TestCase):
     def test_bool_true(self):
         """Check XmlBool true values."""
         for value in ("yes", "true", "1"):
-            node = self._get_node('<node a="%s"/>' % (value,))
+            node = self._get_node(f'<node a="{value}"/>')
             self.assertTrue(manifest_xml.XmlBool(node, "a"))
 
     def test_bool_false(self):
         """Check XmlBool false values."""
         for value in ("no", "false", "0"):
-            node = self._get_node('<node a="%s"/>' % (value,))
+            node = self._get_node(f'<node a="{value}"/>')
             self.assertFalse(manifest_xml.XmlBool(node, "a"))
 
     def test_int_default(self):
@@ -220,7 +220,7 @@ class ValueTests(unittest.TestCase):
     def test_int_good(self):
         """Check XmlInt numeric handling."""
         for value in (-1, 0, 1, 50000):
-            node = self._get_node('<node a="%s"/>' % (value,))
+            node = self._get_node(f'<node a="{value}"/>')
             self.assertEqual(value, manifest_xml.XmlInt(node, "a"))
 
     def test_int_invalid(self):