use function to get wordlists path

The path to wordlist files is hardcoded in the generated sphinx
documentation and because it may differ depending on how the package is
installed this creates different problems:

  - the path hardcoded in the online api documentation may differ from
    the one in each installation, leading to incorrect information
    provided to the user of the api.

  - packaging diceware documentation to specific operating systems may
    generate unreproducible packages.

This commit replaces the variable WORDLISTS_DIR by a function called
get_wordlists_dir() that delays the construction of the path instead of
doing it during module loading.