How about adding a check to `crypt.mksalt()`:

if method and method not in methods:
    raise ValueError(f'method {method} is not supported')

If a method is supplied to `crypt.crypt()`, mksalt() is called with it as an arg, so adding this check will take care of both paths:
crypt(val, method)
crypt(val, mksalt(method))

the only remaining issue is if an (improperly generated) salt is loaded
from somewhere and used to call `crypt()`, but the check above fixes most of the issue.

I can put up a PR if this sounds good.