path parser rejects Path defaults

[timnyborg]

When using env.path to parse a variable, you cannot set the default to a Path variable:

BASE_DIR: Path = Path(__file__).resolve().parent.parent
MEDIA_ROOT: Path = env.path('MEDIA_ROOT', default=BASE_DIR / 'media')

> environs.EnvValidationError: Environment variable "MEDIA_ROOT" invalid: Not a valid string.

Instead you must first cast it back to str:

BASE_DIR: Path = Path(__file__).resolve().parent.parent
MEDIA_ROOT: Path = env.path('MEDIA_ROOT', default=str(BASE_DIR / 'media'))

I think this is a bit counterintuitive, and it works contrary to other parsers like env.dict, which will accept a dict as a default, rather than requiring the string representation of a dict. Happy to try my hand at a preprocess function if it'd be acceptable.

[sloria]

Closed by #231