ok, I guess I should reduce the scope of my statement a bit. I think TypeScript is the better language for writing Pulumi programs. That is, if you and your team have the same level of experience in Python and TypeScript, I think it is wiser to choose TypeScript, mainly because of it's stronger type system.
I totally understand that Python has introduced a type annotation syntax with Python 3.6 and there's mypy as a static type checker, but the whole ecosystem and checker capabilities (and integration of mypy with pulumi) and type system capabilities are just not yet on the level of TypeScript. I would say the maturity and adoption of the Python type system are roughly at the level TypeScript was 3 years ago.
Re Go: I'm actually spending most of my work day writing Go backend code, but it's type system is objectively a lot worse than both Python's 3.6 type annotations and TypeScript's type system and you have to write a lot more code to get things done. For something like Pulumi, where you write a short-lived program that simply expresses / constructs a target-state (and the facto is just programmatically constructing some config that you would otherwise have written in YAML / JSON / TF) all the performance and concurrency features of Go are practically irrelevant (i.e TypeScript, Python is fast enough for this) and Go's verbosity and type system will just make it harder and more time consuming for you to express a valid target state.
A simple example is that in Go functions or structs you cannot easily denote a property as mandatory or optional.
All my statements of course ignore the level of familiarity you and your team has with a particular language, which is of course also important when choosing one language for a project / task.