fix: handle missing environment variables more robustly

stubs/boltons/iterutils.pyi

@@ -0,0 +1,23 @@
+from collections.abc import Callable, Mapping
+from typing import TypeAlias, TypeVar
+
+# Apparently you need Python 3.13 for type var defaults to work. But since this
+# is just a stub file, it's okay if they aren't supported at runtime.
+KIn = TypeVar("KIn")
+KOut = TypeVar("KOut", default=KIn)
+VIn = TypeVar("VIn")
+VOut = TypeVar("VOut", default=VIn)
+
+Path: TypeAlias = tuple[KIn, ...]
+
+# remap() is Complicated and really difficult to define a type for, so I'm not
+# surprised the boltons package doesn't try to type it for you. This particular
+# type declaration works fine for my use of the function, but it's actually
+# vastly more flexible than that - it'll accept any iterable as the root, not
+# just mappings, and you can provide "enter" and "exit" callables to support
+# arbitrary data structures.
+def remap(
+	root: Mapping[KIn, VIn],
+	visit: Callable[[Path[KIn], KIn, VIn], tuple[KOut, VOut] | bool],
+	reraise_visit: bool = False,
+) -> Mapping[KOut, VOut]: ...