adding warning when requests_futures isnt installed

[RichieCahill]

This is a super small pr. I dont think it requires any new tests because of its nature. Please let me know if I did anything wrong.

[dimitarOnGithub]

@adehad / @studioj this one looks good to me as-is, but I wonder if the constructor would be a better place for this warning considering it wouldn't be visible until the code attempts to execute the _fetch_pages method? I think it'd be better if the import, along with the assignment of the async_workers and async_class, happens in the constructor itself, that way you get an instant warning that a requirement is missing.

[RichieCahill]

should we insted of warning if async=true and requests_futures isnt installed just raise an exception

[github-actions[bot]]

Label error. Requires exactly 1 of: bug, enhancement, major, minor, patch, skip-changelog. Found:

[RichieCahill]

@adehad please take a look at this pr i have updated parts of it