MAINT: adapt MotifPssmPattern to modern biopython

[MaozGelbart]

Hi,

Since biopython v1.82, Bio.Align.AlignInfo.PSSM class is deprecated (there's a v1.84 biopython out there already) and Bio.motifs.Motif is favored. Prior to this PR, the mentioned deprecated class serves as the input of the MotifPssmPattern constructor. However, the two helper class methods that load pssm files or alignment use Biopython's create/parse methods that return the recommended Bio.motifs.Motif objects. For example, the following code fails with biopython>=1.82:

from dnachisel import MotifPssmPattern

seqs = ['ACGT', 'ACCT', 'AAGT']
motif_name = "test motif"
MotifPssmPattern.from_sequences(seqs, name=motif_name, relative_threshold=0.9)

The result is the following error: AttributeError: 'PSSM' object has no attribute 'length'

The proposed PR adds a test case, changes the documented input class to MotifPssmPattern to the recommended Bio.motifs.Motif, validates the input as such, and fixes the motif loading logic of from_sequences according to the changes in newer biopython.

The proposed change suggests setting a minimally supported biopython version, but this seems beyond the scope of this PR.

[veghp]

Thank you! The test failed on an unrelated issue, so I'll have to fix that before merging into master.