address Bio.Alphabet removal in Biopython 1.78

[emollier]

Greetings,

In version 1.78, Biopython removed the deprecated Bio.Alphabet module. This patch follows recommendations published on https://biopython.org/wiki/Alphabet to allow Kleborate to run with this Biopython version.

The test suite would otherwise fail with the following backtraces:

======================================================================
ERROR: test.test_genomes (unittest.loader._FailedTest)
----------------------------------------------------------------------
ImportError: Failed to import test module: test.test_genomes
Traceback (most recent call last):
  File "/usr/lib/python3.8/unittest/loader.py", line 436, in _find_test_path
    module = self._get_module_from_name(name)
  File "/usr/lib/python3.8/unittest/loader.py", line 377, in _get_module_from_name
    __import__(name)
  File "/<<PKGBUILDDIR>>/.pybuild/cpython3_3.8/build/test/test_genomes.py", line 18, in <module>
    from kleborate.kleborate import get_data_path, get_output_headers, gunzip_contigs_if_necessary, \
  File "/<<PKGBUILDDIR>>/.pybuild/cpython3_3.8/build/kleborate/kleborate.py", line 28, in <module>
    from .resBLAST import read_class_file, get_res_headers, resblast_one_assembly
  File "/<<PKGBUILDDIR>>/.pybuild/cpython3_3.8/build/kleborate/resBLAST.py", line 25, in <module>
    from .truncation import truncation_check
  File "/<<PKGBUILDDIR>>/.pybuild/cpython3_3.8/build/kleborate/truncation.py", line 16, in <module>
    from Bio.Alphabet import IUPAC
  File "/usr/lib/python3/dist-packages/Bio/Alphabet/__init__.py", line 20, in <module>
    raise ImportError(
ImportError: Bio.Alphabet has been removed from Biopython. In many cases, the alphabet can simply be ignored and removed from scripts. In a few cases, you may need to specify the ``molecule_type`` as an annotation on a SeqRecord for your script to work correctly. Please see https://biopython.org/wiki/Alphabet for more information.

In hope it helps,
Kind Regards,
Étienne.