fix: typing strictness

[daabr]

A few random fixes for type-related warnings from Pylance in VS Code, when type checking mode is "standard" instead of the default "off". Not specific to any project or task, and not a complete fix of the repo, just a few low-hanging fruits.

Most common: str | None --> str in places where it's clear that None is not expected.

os.getenv("var name") --> os.getenv("var name") or "default value"

dict.get("key") --> dict.get("key", "default value")

Also a few import tweaks to fix false-positive type warnings.