Micro-optimize ExpandTypeVisitor by JukkaL · Pull Request #19461 · python/mypy

@JukkaL

Specialize a hot for loop for the concrete `tuple` and `list` types.
Also add a fast path for empty type arguments.

The approach is similar to what I used in #19459.

This is a part of a set of micro-optimizations that improve self check
performance by ~5.5%.

sterliakov

ilevkivskyi