Humm, very odd.
Your code runs just fine on my system: Debian based Linux running Python 3.6.9
Mind, that shebang (that I posted) does not work on my system (maybe it’s for Apple OS or MS Windows) I use #!/usr/bin/python3. That said, your code still runs on my system with #!/usr/bin/python2.
You say…
… but maybe you should let the guys here (most of whom are VERY knowledgeable, much more so than I) be the judge of that.
{content removed: I get so used to posting this, that it’s become 2nd nature}
Sorry I can’t be of more help.
edit: just to be clear, this is what I get on the output
Content-type: text/html
StartingMain