There's nothing to stop the trademark holder from asking you to stop using their trademark.
But they need to ask a court to force you. And these people would lose fair and square. All the Python people would need to do is produce a couple of documents from back in the day in 1995 or whenever, and the Judge would toss the trademark as invalid. Prior use trumps registration! Which is why, with all the software I've tried to start, I always stick a (TM) after the name. It's not a formal registration, but it's a stake in the ground making a claim. And that stake WILL void any formal claim made later by someone else.
The problem is (admittedly to a far lesser extent than in the US) that this whole mess causes uncertainty and expense. Legally, it's extremely clear cut. If Python was using that name before the trademark was registered, then it's up to the trademark holder to prove that they were using it earlier still.
I personally couldn't, but someone could set up a new burger bar called McDonalds and there's nothing the big burger chain could do about it - AS LONG AS the guy setting it up was called McDonald (and hadn't changed his name by deed poll, as this would be seen as an end-run round the rules).