Fix uploading a language pack

[cp2004]

In Python 3, filter() returns an iterable rather than a list. Whilst this is better for performance, our list is not long and it means we can call len() on the list.

Tested using reproduction steps in #3814 and verified that language packs can now be installed properly.

Closes #3814

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What does this PR do and why is it necessary?

Fixes issue uploading language packs on Python 3. This doesn't appear to be a regression in 1.5.0rc1, and a change introduced by 63723ba that removed a change introduced by 30359f5, both claiming to do the same thing.

How was it tested? How can it be tested by the reviewer?

Follow steps outlined in #3814

Any background context you want to provide?

#3814 and #3802 (comment)

What are the relevant tickets if any?

#3814 and #3802 (comment)

Screenshots (if appropriate)

Further notes

Not sure if this counts as a regression, since it has been an issue since 1.4.0 was released, and only when using Python 3. However, since more and more users are moving to Python 3, or installing by default now (both manual and soon OctoPi) I think this should be included in 1.5.0. But at the end of the day, its not up to me. Change the branch to whatever you think works for this fix 🙂