Option 1

I think this works well:

my_env = os.getenv("ENV_VAR", 'False').lower() in ('true', '1', 't')

It allows: things like true, True, TRUE, 1, "1", TrUe, t, T, …

Update: After I read the commentary of Klaas, I updated the original code my_env = bool(os.getenv(... to my_env = os.getenv(... because in will result in a bool type

Option 2

UPDATE:
After the @MattG commentary, I added a new solution that raises an error for entries like ttrue instead of returning False:

# ...
import os
# ...

def get_variable(name: str, default_value: bool | None = None) -> bool:
    true_ = ('true', '1', 't')  # Add more entries if you want, like: `y`, `yes`, `on`, ...
    false_ = ('false', '0', 'f')  # Add more entries if you want, like: `n`, `no`, `off`, ...
    value: str | None = os.getenv(name, None)
    if value is None:
        if default_value is None:
            raise ValueError(f'Variable `{name}` not set!')
        else:
            value = str(default_value)
    if value.lower() not in true_ + false_:
        raise ValueError(f'Invalid value `{value}` for variable `{name}`')
    return value in true_

# ...

my_env1 = get_variable("ENV_VAR1")
my_env2 = get_variable(name="ENV_VAR2") # Raise error if variable was not set
my_env3 = get_variable(name="ENV_VAR3", default_value=False) # return False if variable was not set