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_git_config.py

@@ -100,7 +100,7 @@ class GitConfigReadOnlyTests(unittest.TestCase):
             ("intg", 10737418240),
         )
         for key, value in TESTS:
-            self.assertEqual(value, self.config.GetInt("section.%s" % (key,)))
+            self.assertEqual(value, self.config.GetInt(f"section.{key}"))
 
 
 class GitConfigReadWriteTests(unittest.TestCase):