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>

subcmds/sync.py

@@ -1394,7 +1394,7 @@ later is required to fix a server side protocol bug.
 
             if username and password:
                 manifest_server = manifest_server.replace(
-                    "://", "://%s:%s@" % (username, password), 1
+                    "://", f"://{username}:{password}@", 1
                 )
 
         transport = PersistentTransport(manifest_server)