Biopython is great for working with sequence data in python. Taken almost verbatim from their documentation (https://biopython.org/wiki/SeqIO):

from Bio import SeqIO
import sys

count = SeqIO.convert(sys.argv[1], "genbank", sys.argv[2], "fasta")
print("Converted %i records" % count)

Save as anything, e.g. script.py, then:

python3 script.py example.gb example.fasta