Apply minor import optimizations to Python scripts

[VariantXYZ]

The python scripts for converting tilemaps and tilesets could be a little better in their imports to reduce total build time:

Reference https://discuss.python.org/t/reducing-import-time-over-several-repeated-calls-in-a-makefile/79712/8

We can also use python3 -S instead of python3 in the Makefile to reduce import time by skipping the site module.

[VariantXYZ]

This really only affects clean builds though, iterative builds are a non-issue anyway...