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.