makejson: Use .startswith, not is

More specifically, the use of `is` here appears to be a bug, since that
operator tests for operator identity. The correct operator to use is
`==`, but in this context using `.startswith()` better expresses what
we're actually checking.

scripts/makejson

@@ -34,7 +34,7 @@ json_file = os.getenv("JSON")
 if version is None:
     sys.stderr.write("Version is not specified for release\n")
     sys.exit(1)
-if version[0] is "v":
+if version.startswith("v"):
     # Strip the leading "v" from versioning
     version = version[1:]