Fix bytes vs unicode error when missing DTD
Update Seq.py

Fix the docstring of Seq().count() and MutableSeq().count().
More updated NCBI DTD files.

Adding http://www.ncbi.nlm.nih.gov/corehtml/query/DTD/bookdoc_140101.dtd
and http://www.ncbi.nlm.nih.gov/corehtml/query/DTD/pubmed_140101.dtd
and http://www.ncbi.nlm.nih.gov/corehtml/query/DTD/nlmmedlinecitationset_140101.dtd

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