Sequence parsing broken

[VarIr]

We parse sequences with Biopython.SeqIO, using specific Alphabets. In Bio==1.78 these were removed altogether, which breaks phenotrex.
For now, we can simply require Bio<=1.77.

We could simply move the logic from old Biopython code to phenotrex. It's a matter of a few lines of code. (And improve that awful code, while at it. Just have a look at how _verify_alphabet works...)

On the other hand, the current parsing situation is not optimal, since a protein sequence made only from letters from the ambigous DNA alphabet would be recognized as DNA, with no possibility for the user to override.