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_superproject.py

@@ -34,7 +34,7 @@ class SuperprojectTestCase(unittest.TestCase):
     PARENT_SID_KEY = "GIT_TRACE2_PARENT_SID"
     PARENT_SID_VALUE = "parent_sid"
     SELF_SID_REGEX = r"repo-\d+T\d+Z-.*"
-    FULL_SID_REGEX = r"^%s/%s" % (PARENT_SID_VALUE, SELF_SID_REGEX)
+    FULL_SID_REGEX = rf"^{PARENT_SID_VALUE}/{SELF_SID_REGEX}"
 
     def setUp(self):
         """Set up superproject every time."""