It is a very simple, but you have to find sequence GI instead chromosome number. You can find GI in NCBI's Nucleotide DB.

For example, the mouse chromosome 6 has GI = 307603377, and you want to get a sequence of plus strand from 400100 to 400200:

from Bio import Entrez, SeqIO
Entrez.email = "A.N.Other@example.com"     # Always tell NCBI who you are
handle = Entrez.efetch(db="nucleotide", 
                       id="307603377", 
                       rettype="fasta", 
                       strand=1, 
                       seq_start=4000100, 
                       seq_stop=4000200)
record = SeqIO.read(handle, "fasta")
handle.close()
print record.seq

Parameters description from NCBI's efetch help:

strand - what strand of DNA to show (1 = plus or 2 = minus)
seq_start - show sequence starting from this base number
seq_stop - show sequence ending on this base number
complexity - gi is often a part of a biological blob, containing other gis

Another homework?
Use combination of googling and reading, please. There you are biopython cook book.

I think you can also use EnsEMBL (and NCBI I believe) via the PyCogent toolkit to do this using Python.

Check out http://pycogent.sourceforge.net/ - the examples and cookbook contain some decent code that may be helpful :-)