Here's my 2 cents: Why not use YAML?

Code:

import os, yaml

bool(yaml.load(os.getenv(MY_VAR) or "", yaml.Loader))

Test:

for val, expected in {
    "yes": True,
    "no": False,
    "on": True,
    "off": False,
    "true": True,
    "false": False,
    "1": True,
    "0": False,
    "": False,
    None: False,
}.items():
    actual = bool(yaml.load(val or "", yaml.Loader))
    assert actual == expected