fix(python): strptime now respects pl.Datetime's time_unit

[MarcoGorelli]

closes #6228

Here's my attempt at fixing the linked issue - I'm new to rust so apologies if I've done anything illegal here, I'll study it properly now

[alexander-beedie]

I think you should probably make the new tu param an Option on the Rust side and keep the existing logic in place if no explicit timeunit is supplied - that way it can continue to be auto-inferred if desired (eg: if called from Rust; only the Python side appears to examine the format string now, making it a regression for Rust?)

[ritchie46]

Thank you @MarcoGorelli!