memory: handling of `lru_cache`

[adbar]

See discussion in https://github.com/adbar/htmldate/issues/56.

[adbar]

• [x] check all functions using such a cache
  • [x] in htmldate itself
  • [x] in all the underlying libraries
• [x] write a reset_caches() function which calls function_name.cache_clear() on all concerned functions

[adbar]

Last thing to do: add the new function to the docs before the next release.