PRIME1 As previously submitted
Test files (from the example input)
Some simple optimisations, cut the time by almost half!
Avoiding an int(). Does not make a huge different (as expected)
Saving index (plus one, not to clash with False), instead of True, in the sieve. Eliminates enumerate() and allows filter(). But that makes it slowers (probably the list(range())
Revert "Saving index (plus one, not to clash with False), instead of True, in the sieve. Eliminates enumerate() and allows filter(). But that makes it slowers (probably the list(range())"

This reverts commit 8736919198d7d34f255017d7db97b1ba033ec44d.
Adding lists is expensive(-ish)!
Expressing the start of the sieving differently. This ended up not having an impact on speed, but I like if better this way.