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_project.py

@@ -151,7 +151,7 @@ class CopyLinkTestCase(unittest.TestCase):
                     # "".
                     break
                 result = os.path.exists(path)
-                msg.append("\tos.path.exists(%s): %s" % (path, result))
+                msg.append(f"\tos.path.exists({path}): {result}")
                 if result:
                     msg.append("\tcontents: %r" % os.listdir(path))
                     break